I think Luther is right: “Most necessary it is, therefore, that we should know this article well, teach it unto others, and beat it into their heads continually.”

So here is John Stott to do that:

In these verses an important word occurs for the first time in Galatians. It is central to the message of the Epistle, central to the gospel preached by Paul, and indeed central to Christianity itself. Nobody has understood Christianity who does not understand this word. It is the word ‘justified’. The verb comes three times in verse 16 and once in verse 17, while the noun ‘justification’ occurs in verse 21.
In this paragraph, then, Paul unfolds the great doctrine of justification by faith. It is the good news that sinful men and women may be brought into acceptance with God, not because of their works, but through a simple act of trust in Jesus Christ. Of this doctrine Martin Luther writes: ‘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.’ In other places he refers to it as the ‘chief’, the ‘chiefest’ and ‘the most principal and special article of Christian doctrine’, for it is this doctrine ‘which maketh true Christians indeed’. He adds: ‘if the article of justification be once lost, then is all true Christian doctrine lost.’
Similarly, Cranmer wrote in the first Book of Homilies, ‘This faith the Holy Scripture teacheth: this is the strong rock and foundation of Christian religion: this doctrine all old and ancient authors of Christ’s Church do approve: this doctrine advanceth and setteth forth the true glory of Christ, and beateth down the vain glory of man: this whosoever denieth is not to be counted for a true Christian man, nor for a setter forth of Christ’s glory, but for an adversary of Christ and His gospel, and for a setter forth of man’s vain glory.’
If the doctrine of justification is central to the Christian Religion, it is vital that we understand it. What does it mean? ‘Justification’ is a legal term, borrowed from the law courts. It is the exact opposite of ‘condemnation’ (Cf. Dt.25:1; Pr.17:15; Rom.8:33,34). ‘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 favour by which He puts a sinner right with Himself, not only pardoning or acquitting him, but accepting him and treating him as righteous.
Many people find Paul’s language alien to their vocabulary, and his argument intricate and complex. But is Paul not writing about a universal human need, as pressing today as it was 2000 years ago? For there are at least two basic things which we know for certain. The first is that God is righteous; the second is that we are not. And if we put these two truths together, they explain our human predicament, of which our conscience and experience have already told us, namely that something is wrong between us and God. Instead of harmony there is friction. We are under the judgment, the just sentence, of God. We are alienated from His fellowship and banished from His presence, for ‘what partnership have righteousness and iniquity?’ (2 Cor. 6:14).
This being so, the most urgent question facing us is the one which Bildad the Shuhite asked centuries ago, ‘How then can man be righteous before God?’ (Jb. 25:4). Or, as Paul would put it, ‘How can a condemned sinner be justified?’ His answer to these crucial questions is in this paragraph. First, he expounds the doctrine of justification through faith (verses 15,16). Then he argues it (verses 17-21), dealing with the commonest objection to it and demonstrating the utter impossibility of any alternative.