If Romans 16 was sent to Ephesus, then the part sent to Rome might have originally ended at 15:33: “The God of peace be with you all. Amen.” Interestingly, the earliest copy of Paul’s letters, which dates to around AD200, has the doxology of Romans 16:25-27 exactly here, at the end of Romans 15. This location may indicate that some very early copies of Romans ended there. Early manuscripts of Romans are all over the map in how they end. Many early ones have this doxology right after Romans 16, as it is in our Bibles. Others have it both here and at the end of chapter 14! Some do not have it after chapter 16 but have it at the end of chapter 14. Then of course some do not have it at all.
The originality of the doxology itself faces serious questions. It does not sound like the rest of Romans in many respects (e.g., “the revelation of the mystery hidden for long ages past, but now revealed and made known through the prophetic writings by the command of the eternal God”). It does not appear in all manuscripts and widely differs in where it is placed in those that have it. To top off things, some manuscripts of Romans have 16:20b after verse 23 to become verse 24: “The grace of our Lord Jesus be with you.” It is as if they are hinting that earlier copies of Romans ended here and did not have the doxology. Most experts accordingly do not believe this doxology was originally a part of Romans.
Here is what we wonder. The letter that Paul sent to Rome ended with Romans 15:33. Romans 16 was sent to Ephesus as a letter of recommendation for Phoebe and a greeting of the churches there. Meanwhile, the early Christian Origen (ca. AD200) implies that a man named Marcion (ca. AD150) had lopped off the last two chapters. Therefore, near the end of the 100s AD, we found three versions of Romans out there: one that ended after chapter 14, one that ended after chapter 15, and one that ended after chapter 16. All three of these seemed to lack a proper ending. In some manuscripts, Romans 16:20b was moved to the end of 16:23 to give a more appropriate ending. Meanwhile, someone created the doxology, and it found its way on to all the different versions of Romans.