Third, he is talking to a GENTILE congregation (see Romans 15:15-25) who had only the law of conscience and were never under the law of Moses.
First, the Romans 15 text does not really support your position, when understood in a broader context. Paul is indeed setting himself in the position of being the apostle to the Gentiles (and thereby setting himself in contrast to Peter). But this does not mean that the letter is written only to Gentiles. In fact, the evidence is strong against such a position.
Here is a summary overview for the case that the book of Romans was written to a church at Rome populated by at least some Jews:
1. The notion that the Roman church was an exclusively Gentile one is at odds with the historical record. Jews were expelled from Rome in 49 AD by Claudius. However, when Claudius died in 54 AD, Jews returned to Rome. The majority of scholars writing on Romans propose the letter was written in late 55/early 56 or late 56/early 57 (Wikipedia). Therefore, there is strong historical plausibility that when Paul wrote Romans, there were indeed Jews in the church at Rome.
2. Here in chapter 2, Paul is clearly writing to a Jew:
You who boast in the Law, through your breaking the Law, do you dishonor God? For “THE NAME OF GOD IS BLASPHEMED AMONG THE GENTILES BECAUSE OF YOU….” Clearly, Paul is addressing such a rebuke to a specifically Jewish reader.
3. In Romans 7, Paul makes
this statement:
But now we have been released from the Law, having died to that by which we were bound, so that we serve in newness of the Spirit and not in oldness of the letter. I suggest it is manifestly clear that this
must be the Law of Moses. And who is subject to the Law of Moses? Cetainly not the Gentile. So Paul must be addressing a
Jew, at least in this particular case. Do you challenge that the “Law” here is the Law of Moses? That would really stretch credulity. Note that a few breaths later, Paul refers to the “coming of the law” and gives us the commandment “do not covet”. Clearly, this is an allusion to the delivery of the Law of Moses, at Mount Sinai, including the 10 commandments. Besides, to what other
written (“oldness of the latter) law could Paul possibly be referring? The US Constitution?
4. In Romans 13, Paul writes this:
Owe nothing to anyone except to love one another; for he who loves his neighbor has fulfilled the law. 9 For this, “YOU SHALL NOT COMMIT ADULTERY, YOU SHALL NOT MURDER, YOU SHALL NOT STEAL, YOU SHALL NOT COVET,…” Here there is no doubt at all – Paul is talking about the Law of Moses. The argument here is a little more subtle. What would have to be the case in order for Paul to have written these particular words to an exclusively
Gentile Roman church? Obviously, it would have to be the case that such Gentiles were thinking in terms of fulfilling the Law of Moses! But that makes no sense – the Gentile knew full well that the Law of Moses was given to Jews only. So Paul must be addressing at least
some Jews here, even though, of course, his injunction to “love one another” is clearly directed
all believers.
There are other arguments to be made that the church that Paul is writing to must have contained some Jews.