Justification is an instantaneous legal act of God in which He thinks of our sins as forgiven and Christ’s righteousness as belonging to us, and declares us to be righteous in His sight.

A right understanding of justification is absolutely crucial to the whole Christian faith.  Once Martin Luther realized the truth of justification by faith alone, he became a Christian and overflowed with the new-found joy of the gospel.  The primary issue in the Protestant Reformation was a dispute with the Roman Catholic Church over justification.  If we are to safeguard the truth of the gospel for future generations, we must understand the truth of justification.  Even today, a true view of justification is the dividing line between the biblical gospel of salvation by faith alone and all false gospels of salvation based on good works.
– Wayne Grudem

Justification signifies a reckoning as just or righteous.  It is a legal concept; the person who is ‘justified’ is the one who gets the verdict in a court of law.  Used in a religious sense it means the getting of a favorable verdict before God on judgment day.  To many it seemed that this was done by doing good deeds; to Paul it was crystal clear that no one can do this, for we are all sinners.
– Leon Morris

This is the truth of the gospel.  It is also the principal article of all Christian doctrine, wherein the knowledge of all godliness consisteth.  Most necessary it is, therefore, that we should know this article well, teach it unto others, and beat it into their heads continually. 
– Martin Luther

Justification is a legal term, borrowed from the law courts.  It is the exact opposite of ‘condemnation’.  ‘To condemn’ is to declare somebody guilty; ‘to justify’ is to declare him not guilty, innocent or righteous.  In the Bible it refers to God’s act of unmerited favor by which He puts a sinner right with Himself, not only pardoning or acquitting him, but accepting him and treating him as righteous.

Justification by works was the position of the Jew and the Judaizer.  Paul describes them as “seeking to establish their own righteousness” (Rom. 10:3).  It has been the religion of the ordinary man both before and since.  It is the religion of the man-in-the-street today.  Indeed, it is the fundamental principle of every religious and moral system in the world except New Testament Christianity.  It is popular because it is flattering.  It tells a man that if he will only pull his socks up a bit higher and try a bit harder, he will succeed in winning his own salvation.

But it is all a fearful delusion.  It is the biggest lie of the biggest liar the world has ever known, the devil, whom Jesus called ‘the father of lies’.  Nobody has ever been justified by the works of the Law, for the simple reason that nobody has ever perfectly kept the Law.  The works of the Law, a strict adherence to its demands, are beyond us.  We may keep some of the Law’s requirements outwardly, but no man except Jesus Christ has ever kept them all.  Indeed, if we look into our hearts, read our thoughts and examine our motives, we find that we have broken all God’s laws.  For Jesus said that murderous thoughts make us murderers, and adulterous thoughts make us adulterers.  No wonder the Scripture tells us: ‘by works of the Law shall no flesh be justified’.  The astonishing thing is that anybody has ever imagined he could get to God and to heaven that way.  – John Stott

Paul turns from his rebuke of Peter to again address the Galatians (assuming the quote to Peter ends at verse 14).  He now passionately defends the gospel message of justification by grace alone through faith alone.  This is the key passage of the book.  It is the turning point from biography to doctrine, but it also establishes the whole premise of his writing and the main thesis of the letter (and really the basis of the gospel).  The remaining chapters of the letter simply elaborate on the tenets of this passage.

15-16
In verses 15 and 16 Paul builds a logical case as to why Peter’s behavior made no sense.  While he and Paul are both Jews by birth brought up under the Law (not pagan Gentiles – sinners – who had no law to restrain them), they also acknowledged (“nevertheless knowing…”) the Law cannot save when they believed in Christ.  To believe in Christ is to agree that justification cannot come from works of the Law; otherwise Christ did not need to come and there is really nothing to believe in.  Since Christ came and they believed in Him for salvation, they obviously realize that the works of the Law cannot save.  That being the case, why try to keep the Law now – or insist that the Gentiles do – since by the works of the Law shall no flesh be justified? 

Paul effectively repeats this premise three times in verse 16.  He does not want his readers to miss it as nothing is more important to their salvation.  Justification is ONLY through Christ.  He applies a slightly different nuance to each statement.  “We know that a man (general) is not justified by the works of the Law but through faith in Christ Jesus.  Consequently, we (personal) believed in Christ Jesus so we would be justified by faith in Him and not by works of the Law – since the works of the Law do not justify anyone (universal).”  Note that he says the reason they believed was to be justified – we have believed in Christ Jesus, that we may be justified by faith in Christ.  Belief – faith – leads to justification.  Nothing else does.

17-19
Verse 17 is difficult to translate and thus several interpretations exist.  Two possible meanings are presented here and both have reasons to recommend them.

The first is that verse 17 builds on the logic of 15-16.  If we who are seeking to be justified in Christ (what we have just established we are doing) require others to obey the Law as part of their salvation, does that mean Christ is a minister of sin since He leads believers away from the Law?  Do we not make Christ exactly that by telling others the Law is required when Christ’s gospel teaches that it is not?  But can Christ possibly be an agent of sin?  May it never be!

A second interpretation is that verse 17 clears up a possible misunderstanding of Paul’s message of justification apart from works.  If our works do not matter and have no bearing on our justification, does that mean Christ effectively encourages sin by making the Law meaningless?  Does Christ take away the motivation for the Jews to follow the Law (and make them just like the Gentile sinners – same word as in verse 15) thereby becoming a minister of sin?  Paul’s answer to this thinking – May it never be!

Notice how Paul describes the believer’s state – he is justified in Christ.  Justification is through Christ’s work alone and leads to the unity of the believer with Christ.  We do not simply believe in Christ but we actually are in Christ.  As such, it is impossible for the child of God to repudiate the morality of the Law for a life of sin.  We are joined with One who is perfect and we live a new life in Him.  We may be sinners in the eyes of legalists who look to the Law for justification, but we do not live the sinful lives of lawless people.  Thus our Savior is not a minister of sin.

The real minister of sin is the one who calls people back to the Law after they believe in Christ for justification.  Verse 18 explains 17 (note that it begins with for).  If Paul goes back to the Law – if I rebuild what I have once destroyed – then HE is the transgressor because he violates the message of Christ.  The gospel destroys the theology that claims one can work his way to God through obedience to the Law.  To rebuild what the gospel destroyed is to truly sin (and is exactly what Peter did when he refused to dine with the Gentiles).

The reason it is a transgression is that Paul died to the Law – verse 19 (note that it too begins with for).  “How can I tell Gentile believers to obey the Law when I myself died to the Law?”  He had to die to the Law in order that he might live to God.  Just as he believed in Christ that he might to be justified (16), so he died to the Law that he might live to God.  It is impossible to look to the Law for justification and live in Christ.  It is impossible to believe in works and enjoy the grace of God.   

He says he died to the Law through the Law.  This means the Law had its intended purpose in Paul’s life.  It showed him the futility of trying to earn his way to God.  The Law is impossible to keep, thus it points the failed adherent to another answer.  If the Law is impossible there must be a different way.  Without the Law the believer does not understand how pointless it is to come to God in his own righteousness.  Through the Law he finds the way to Christ.

Consider: To die to the Law is really to die to self.  The Law represents the theology that says my justification depends on me.  It is about me and my efforts, not God.  But when I accept the gospel that says it is not about me at all, I die to me.  I die to me and live to God.  And I continue to die to me as I live out my salvation.  If my efforts did not bring me to the throne it logically follows that my efforts cannot keep me before the throne.  My justification is not about me and my life with Christ is not about me.  I die to me continually as I remember the gospel that tells me I had nothing to do with my justification.  And this completely changes my expectations of a life that is NOT ABOUT ME.

20
Verse 20 puts everything together.  This is the result of believing in Christ alone for justification.  This is living in Christ.  This is what it means to die to the Law and live to God.

When I believe in Christ for justification I become so identified with His death that I am counted as being with Him on the cross – I have been crucified with Christ.  I die with Him (notice the tense – have been – in one sense I continually crucify myself with Him) and I crucify my sin and my reliance on works and my old life entirely.  I die.  ME dies.  Any self-based claim to righteousness dies.  I die to the law.  I die to sin.  The penalty for my sin is paid.  God can rightly declare me His.

[Which brings up a vital point about justification.  Justification is not simply a declaration – it is a declaration based on the satisfaction of justice.  God does not simply declare us righteous out of His good nature.  He declares us righteous because the penalty for our sins was paid.  Christ’s death satisfied God’s perfect call for justice.  God can justly call us righteous because Christ took our place on the cross.  God does not simply pardon us or forgive us – He declares us righteous because of Christ’s death.  Justification is based on the cross.]

Believing in Christ does not only identify me with His death.  When I die with Christ I no longer live as I did before.  I actually no longer live at all (because I died!) but Christ lives in me.  I continue to physically live (the life I now live in the flesh) but I now live by faith in the Son of God.  This is what it means to live to God.  This is what it means to be in Christ.  My justification is not about me and my ongoing life is not about me.  It is all about faith in the Son of God.  Faith brings me to the throne and faith keeps me before the throne.  It is only through Christ that I am accepted.

The context of Galatians 2:20 is justification – being declared righteous on the basis of the righteousness of Christ.  So when Paul wrote that he lived by faith in the Son of God, he was not in that passage referring to a dependence on Christ for spiritual strength (as is the case in Philippians 4:13), but to a dependence on Him for his righteous standing before God on a day-to-day basis.

It is important to realize that we were not only saved by faith at a particular point in time, but we are to live by faith in Christ every day of our lives.  This means that I must continue to renounce any confidence in my own goodness and place my confidence solely in Christ every day of my life, not only for my eternal salvation, but for my daily acceptance before a holy God. 
– Jerry Bridges, The Discipline of Grace.

To live by faith in the Son of God is how I live my life as a new creature.  I am new because I died and rose again with Christ.  My old life is dead and my new life is not my own.  I am dead to the theology of works and dead to self-reliance and dead to the slavery of sin and alive only to faith in Christ.  He lives IN ME and thus I can in no way live how I lived before my crucifixion.  Therefore if any man is in Christ, he is a new creature; the old things passed away; behold, new things have come – II Cor 5:17. 

How is this new life possible?  Because He loved me and delivered Himself up for me.  Paul makes this very personal.  Who did Jesus love?  ME.  Who did He deliver Himself up for (note that no one delivered Him – HE delivered HIMSELF)?  ME.  I have a new life that is not my own and I have been counted as crucified and the penalty for my sins has been paid and I am counted fully righteous before God because HE LOVED ME AND DELIVERED HIMSELF UP FOR ME. 

Thought: There really is not a moment of any day where this should not be at the forefront of our minds.  There is not a problem in the world that cannot be overwhelmed with this truth.  We should continually ask our Father for forgiveness whenever we take this for granted. 

Remembering these principles is the essence of preaching the gospel to ourselves.  We died with Christ, we live through Christ, we are wholly dependent on Christ.  We did not earn our salvation and as believers we do not earn our standing before God.  Christ’s death and resurrection are sufficient for all of salvation and all of life.  Apart from Him we are nothing and in Him is our only hope.  There is no sin His death did not cover (thus there is no reason to ever hesitate to confess the sin in our lives) and on our best days we cannot justify our place before His throne.  Since we and our old lives died with Him we cannot live as we did before our crucifixion.  We must go to the cross for the sobering reminder of what our sin is and to the empty tomb for the joyous reminder of the power available to us.  We must look at the world and its temptations through the truth that He loved me and delivered Himself up for me.  We cannot live a God-pleasing life without continually meditating on the gospel.  In the words of Martin Luther, we must beat it into (our) heads continually.

21
Verse 21 summarizes the logic of Paul’s argument in this text.  To claim salvation by adherence to the Law is to nullify the grace of God.  Grace is not needed if salvation is earned since grace by definition is unmerited.  So it is one or the other – either salvation comes through works of the Law or it comes through God’s grace based on Christ’s work.  It cannot be both as they are mutually exclusive.  And if it comes through the Law then Christ died needlessly – there was no point to a death that did not change anything.

Paul throws down the gauntlet by making his opponents face the logical end to their thinking.  They nullify God’s grace by their claim that Gentiles must keep the Law which then makes Christ’s death superfluous.  This is likely far beyond what they have preached and beyond what they intend.  But it is, in fact, what their theology results in.  They now must counter the conclusion of their own teaching.  And more than simply debating the merits and applicability of the Law they must now defend their position on Christ’s death.  This verse establishes why Paul has been so vehement in his defense of the gospel (1:8-9, 2:5,14) – if any other message is true then Christ died needlessly.

Closing Thoughts
We must keep in mind that our justification by God is based solely on the meritorious work of Christ and our union with Him.  That is, God sees us legally as so connected with Christ that what He did, we did.  When He lived a life of perfect obedience, it is as if we had lived a life of perfect obedience.  When He died on the cross to satisfy the just demands of God’s Law, it is just as if we had died on that cross.  Christ stood in our place as our representative, both in His sinless life and His sin-bearing death.  This is what Paul referred to when he said, “I have been crucified with Christ.”
 – Jerry Bridges, The Discipline of Grace.

There is a new “I”—I do still live. But look who it is. It is no longer an “I” who craves self-reliance or self-confidence or self-direction or self-exaltation.  The new “I” looks away from itself and trusts in the Son of God, whose love and power was proved at Calvary. From the moment you wake in the morning till the moment you fall asleep at night, the new “I” of faith despairs of itself and looks to Christ for protection and the motivation, courage, direction, and enablement to walk in joy and peace and righteousness. What a great way to live! 
– John Piper