Calvin’s commentary on Habakkuk 2.4 is a great snapshot of what the Reformation was/is all about. He mentions how the NT relates to the OT (Paul to Habakkuk), he makes exegetical conclusions based on the Hebrew text, he points out the terrible errors of Rome, he talks about the law/gospel distinction, he shows how doctrine relates to comfort, and he ends the discussion with an excellent gospel centered prayer of hope. He also mentions what have been come to be known as the solas, specifically sola fide, sola gratia, and solus Christus. Here are a few quotes on these topics.
“…the Prophet [Habakkuk] understands by the word amunat that faith which strips us of all arrogance, and leads us naked and needy to God, that we may seek salvation from him alone, which would otherwise be removed from us.”
“Paul [in Romans 1.17] very rightly connects these things together – that righteousness is made known in the Gospel – and that it comes to us by faith only. …Paul assumes that these, even faith and law, are contrary, the one to the other; contrary as to the work of justifying…as to justification, the law accords not with the gospel any more than light with darkness.”
“But we obtain righteousness by faith alone for this reason, because God finds nothing in us which he can approve, or what may avail to obtain righteousness. … If righteousness be of faith, then it is of grace alone, and if by grace alone, then it cannot be by works. … As then, faith acquires for us favor before God, and by this favor we are counted just, so all works must necessarily fall to the ground, when righteousness is ascribed to faith.”
Of course, there is a lot more to his discussion – these are just a few great quotes. In fact, I recommend finding it online (if you don’t own the commentary), printing it out, and spending ten or twenty minutes reading through it. You’ll be encouraged in the Christian faith and reminded not only of the necessity of the Reformation, but also of the beauty of the doctrines of grace.