First is his audience.
7 To all who are in Rome, beloved of God, called to be saints:
13 Now I do not want you to be unaware, brethren, that I often planned to come to you (but was hindered until now), that I might have some fruit among you also, just as among the other Gentiles.
They are gentiles in Rome. Gentiles mean those that are not Israel. But they were gentiles that were proselytized to Israel as he says here:
17 Indeed you are called a Jew, and rest on the law, and make your boast in God,
So these gentiles are going to be familiar to the promises made to Israel. They came to God through Israel, exalting her and keeping of the law. What Paul wants to do is share his gospel with these gentiles. His gospel is not the same as Israel’s good news. They have the same savior but a different ministry and mission. They in Rome knew they were to show their faith by their works. They did not have his good news.
15 So, as much as is in me, I am ready to preach the gospel to you who are in Rome also.
The audience is gentiles that were proselytes to Israel, they know of the law and they know they are required to keep it. They are called Jew as a result. And Paul is now ready to share his gospel with them.
In his opening address to the Romans Paul writes that they are called to be saints.
Romans 1:7 KJV To all that be in Rome, beloved of God,
called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ.
What I believe about "called to be saints": I don't take the Calvinist perverted twist that some were chosen for heaven and some for hell, but rather as Jesus Christ was sent in M, M, L and John to none, but the lost sheep of the house of Israel (Matthew 15:24 KJV) as a minister of the circumcision (Romans 15:8 KJV) so we see in the Acts period that Paul was sent (Acts 26:17 KJV) to them which were the called at that time, both Jews and Greeks.
Acts 13:26 KJV Men and brethren, children of the stock of Abraham, and whosoever among you feareth God, to you is the word of this salvation sent.
1 Corinthians 1:24 KJV But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.
There was a remnant which God foreknew at that present time that Paul was gathering into the Body of Christ (Romans 11:1-5 KJV).
The Romans had a faith, but not the mutual faith of both them and Paul. Paul longed to see them that he may impart unto them some spiritual gift, to the end ye may be established (Romans 1:10-12 KJV)
Paul writes that he is ready to preach the gospel to them (that makes no sense if they already trusted after hearing and believing),
Romans 1:15 KJV So, as much as in me is, I am ready to preach the gospel to you that are at Rome also.
as his my gospel is what would "stablish" them into the Body of Christ.
Romans 16:25 KJV Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,