Paul's longstanding policy regarding preaching is summarized in Romans 15:20:

"It has always been my ambition to preach the gospel where Christ was not known, so that I would not be building on someone else's foundation."

So why did he choose to write his most comprehensive statement about the gospel to an ecclesia which at the time he had never visited?

The Roman church contained the most volatile mix of members imaginable in the first century. There were many Jews, still quite committed to the observance of the Law of Moses in many particulars. And there was also a large number of Gentiles, converts from the gross idolatry of the Roman world. In Rome such Gentiles were, in a number of cases, from influential Roman families.

Harry Whittaker describes what he calls "the strange social situation" in Rome:

The Jewish Christians had lived all their days following the strict food laws and prohibitions which all through the centuries have done more than anything else to maintain Jewish separatism. These Jews would inevitably be infected with the spiritual snobbery which was very much a characteristic of Jewry at that time… They knew themselves to be the chosen race, "beloved for the fathers' sakes". [To these Jews] Gentiles were mere "dogs", ignorant idolaters, steeped in all kinds of abomination. Even believing Gentiles would be seen to be religiously much inferior to themselves, and accordingly many (or most) of these Jewish believers would look down on their Gentile brethren, who had not grown up in the nurture of the Scriptures, as though they were of lower status in the Faith.

On the other hand some of these Gentiles were from some of the highest families in the Empire. And practically all of them would be accustomed to thinking of themselves as members of the Master Race, the nation which had built the strongest Empire the world had ever known. Especially were they used to looking down their noses at Jews who [had] somehow managed to get themselves disliked by most Gentiles. Here, then, was a situation which from earliest days was fraught with difficulty. Oil and water do not mix. Then how much less the hope that Jews and Romans would join together in a fullness of fellowship so as to adorn the doctrine of Christ!

Bible Studies, p. 304

It was in this ecclesial environment that Aquila the Jew and Priscilla the Roman lived. They may have seen real evidence of the Body of Christ being split down the middle, into two cliques organized according to nationality. What could be done about it? Since this married couple represented, between themselves, both sides of the great divide, and since they had been close associates and coworkers with the apostle Paul, they may have decided on a course of action designed to bring the Roman ecclesia or church closer together as One Body: They would request that Paul write a letter to the Roman brethren, which would lay out the full gospel of Jesus Christ, but would also present that gospel in the context of the significant differences between the two major components of their ecclesia (cf Rom 9-11), along with strong exhortation to develop and maintain a true unity in the bond of the Spirit (Rom 14-15:13).

Evidence of this background and origin of the Epistle to the Romans is found as early as its introductory statement of intention:

"I am not ashamed of the gospel, because it is the power of God for the salvation of everyone who believes: first for the Jew, then for the Gentile" (Rom 1:16).

Both Jews and Gentiles were in the same situation regarding this salvation: they were all sinners, and all in need of the mercy of God (Rom 1:18-3:20). How could either group look down upon or feel superior to the other? All alike needed the grace of God in equal portions:

"Is God the God of Jews only? Is he not the God of Gentiles too? Yes, of Gentiles too, since there is only one God, who will justify the circumcised by faith and the uncircumcised through that same faith" (Rom 3:29,30).

Going on in the letter, Paul writes of "Abraham, our forefather" (Rom 4:1), but immediately makes it plain that Abraham is the father not just of Jews, but especially of all who have faith. In fact, he is destined to become "the father of many nations" (v 17) — Jews of course but also many, many Gentiles.

On and on Paul leads his readers, through baptism and a new life and the great joys of belonging to the family of God in Christ (Rom 6-8). These were what all believers — no matter their ethnicity or language or background — held in common with one another. And thus to the section mentioned above (Rom 9-11), where the respective current situations of Jews and Gentiles is analyzed, with the assurance that, no matter how far away any peoples may remove themselves from the mercies of God, He is able to graft them into His own "tree" and the One Hope again.

Then to Romans 12:3-8, and the wonderful allegory of the One Body (which Paul developed even further in 1 Corinthians 12), with all its components belonging to one another and working together for the mutual benefit of the whole. And finally to the powerful and practical exhortations in the last major section of the letter (Rom 12-15).

At the end of the chapter cited above, Harry Whittaker concludes: "It makes a very impressive exercise for the modern reader to [read through] the epistle to the Romans at one (or, at most, two) sittings, looking at each paragraph from the point of view suggested here. The coherence and practical value of every section becomes greatly enhanced… Is there a lesson for modern times in all this?"