Starting over.....
I' for one, as I have already posted, would not say "justification by faith" the "article" upon which the Church stands or falls. Justification by faith is, however, a doctrine of the Reformation that the Reformers held to be foundational in response to the Roman Catholic Church's teachings works can and are salvific (beginning with but not limited to, the practice of selling indulgences and the "work" of purchasing them).
The Church is simply those people who have been called out by God from the world into His service through His divinely initiated and maintained Christological covenant, Jesus. The Church is the body of Christ. That is the article upon which the Church stands or falls. That is why sound doctrine matters, because any doctrine that doesn't have Jesus as the sole foundation upon which anyone can salvifically build is a bad doctrine. This is clearly articulated in 1 Corinthians 3:11-15.
1 Corinthians 3:10-15
According to the grace of God which was given to me, like a wise master builder I laid a foundation, and another is building on it. But each man must be careful how he builds on it. For no man can lay a foundation other than the one which is laid, which is Jesus Christ. Now if any man builds on the foundation with gold, silver, precious stones, wood, hay, straw, each man's work will become evident; for the day will show it because it is to be revealed with fire, and the fire itself will test the quality of each man's work. If any man's work which he has built on it remains, he will receive a reward. If any man's work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire.
Technically, justification by faith (alone) would be something built on the foundation of Christ crucified and resurrected.
Which brings me to the next point: Logically speaking, if we were to approach the Pauline epistles based on their comparative content alone then it seems reasonable to conclude the first letter to the Church in Corinth precedes the letter to Romans. How could it be that Paul would write of an "article" he hadn't yet established and then later comment on it as if he were filling in some unwitting deficit or covering a neglected base? Blessedly, that is not how the chronology of the Pauline epistles is decided so the question asked is framed in a red herring. Logically speaking, It is irrational to ask why someone did not do something, especially if that individual has not explained his own "why." Any answer would be inherently and inescapably speculative.
Lastly, for now, the book of Romans is not the foundational to the article of justification in and of itself. Just as no single verse, nor any one single chapter, in any book of the Bible stands alone, so too the letter to the Romans does not stand in isolation of itself. This is especially the case since Paul chronically quotes, cites, and indirectly references the Old Testament throughout the letter. His exposition on justification is consistent with that practice. Someone picking up the Romans epistle for the first time could not correctly comprehend the whole of Paul's case without necessarily consulting the Old Testament upon which Paul built his commentary. In point of fact, historically speaking, it was that very practice on inconsistent exegesis that caused the errors in correctly understanding justification by faith.
There is, of course, an irony to the op because in our Bible the book of Romans is the first epistle. It is the first of the epistles in the ordering of the epistles because in the era when the Bible was compiled the general practice was to place the largest works of a genre first and the smallest ones last. Since Romans is the longest of the epistles, it is placed first in the section of the New Testament that contains the epistles. This is also why Matthew is the first gospel we read. Someone picking up a Bible and starting their reading at the epistolary would read Romans first.