What provoked Paul to write the epistle to the Romans? Scripture and history help us paint a background to the book.
The church was probably founded by Jewish Christians who were baptized in Jerusalem at Pentecost (Acts 2:10). The new Christians most likely faced the same opposition from Jewish leaders that Paul faced throughout the Roman world. The arguments apparently became so heated that, in 49 A.D., Emperor Claudius expelled all of the Jews from Rome to put a stop to it. However, when Nero ascended to the throne in 54 A.D., he was persuaded by his wife to allow the Jews to return. Yet, when the Jewish Christians returned, some of which may have even helped to establish the church, they found the Gentile Christians large and in charge. Undoubtedly, this would have created some tensions and Paul addresses the problem in his Epistle.
Paul also wanted to let the Romans know that the great "apostle to the Gentiles" had not been snubbing them by not visiting them as of yet (1:13-15) and also wanted their assistance in going to Spain on a missionary journey (15:23-24).
The confluence of all of these events, along the urging of the Holy Spirit, provoked Paul to dictate a grand letter to a fellow Christian named Tertius (16:22) around 58 A.D. and 1950 years later it continues to challenge and form Christians. Praise be to God.