To me, the Epistle to the Romans is a job application, albeit an unsolicited one. Paul spends the first chapter winning the trust of the Romans, assuring them that he would fit into their group. Paul begins with a lengthy opening address no doubt designed to impress the Roman Christians that he was, at the same time, sincere and unassuming, and that his teachings about Jesus Christ were in accordance with their own. Paul hopes to preach in Rome for a while (1:15), and needs their acceptance. Romans chapter 15 is an assurance that the stopover will be brief and that he does not intend to step on any toes, before proceeding to Spain. In this regard:
Romans 15:14: And I myself also am persuaded of you, my brethren, that ye also are full of goodness, filled with all knowledge, able also to admonish one another.
In this context, what follows should not be interpreted as Paul writing something to the Romans that they did not already know. Although he had never met the Christians of Rome, he knew they already had faith in Jesus and already believed the gospel. He wanted to show them that he was on the same page.
In verse 16, Paul says he is not ashamed to proclaim the gospel, which he tells the Romans is the power of God for the salvation of everyone who believes: Jew first, and then Greek (Gentile). Verse 1:17 then echoes Habakkuk 2:4 (and possibly Psalm 98), concluding with the citation from Habakkuk 2:4, in Romans 1:17b:
Romans 1:17b: as it is written, The just shall live by faith.
Matthew Poole says this apostle seems to delight in repetitions such as as 'from faith to faith', and there is an elegancy in them. He says the words are variously interpreted and gives two examples:
- from the faith of the Old Testament to the faith of the New
- from a lesser faith to a greater; not noting two faiths, but one and the same faith increasing to perfection.
To extrapolate from Poole's commentary, some theologians are merely speculating as to the meaning of this phrase, seeking to find in it something that suits their own theology.
Steve Moyise makes a reasonable point in 'Quotations', published in As It Is Written, page 21, that it is extremely unlikely that Paul first formulated his gospel as the revelation of God's righteousness 'of' or 'from' faith and only later discovered that Hab 2:4 is the only text in the whole of scripture to make such a connection. It is much more likely that Paul began with the Habakkuk text and formulated his doctrine accordingly.
That being the case we can say that, to a significant extent, Paul's message is defined not by Habakkuk's message, but by Habakkuk's words, which Paul paraphrases but can only change so much. And because Paul did not start from an an intended statement that he found Habakkuk supported, but started from Habakkuk, we should not read too much into how he used words, but keep our focus on the broader message.
Moyise (ibid, page 18) says that, since Paul is introducing himself to the Roman church, he cites a text that he knows (or thinks he knows) will be common ground, and in this way he will gain their confidence. He has deftly altered the meaning of Habakkuk's words and mingled his own words with those of Habakkuk in order to give the impression that Habakkuk means what Paul means.
The precise meaning, as intended by Paul, of "from faith to faith" is obscure, and perhaps that is how Paul intended to leave it. If Matthew Poole is correct when he says Paul delighted in repetitions like this one, then Paul was using this particular construct because it delighted him. More broadly, he wanted to talk about faith, and he wanted to demonstrate that his teachings were well grounded in scripture by citing a passage that would resonate with the Romans.