Romans 1:5, 1:16
New American Standard Bible
5 through whom we have received grace and apostleship to bring about the obedience of faith among all the Gentiles in behalf of His name,
16 For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.
In this scripture, Paul is referring to Jesus Christ - in a letter to the Christians in Rome
He is claiming to be a direct representative of him
And says that the mission Christ gave him is to spread the faith amongst the gentiles (as opposed to the Jews)
So, basically he is saying "Jesus has sent me to convert the gentiles!"
Paul is attempting to involve the Christians of Rome in this endeavour
Clearly he thinks of Christianity as being distinct from Judaism although he believes he can extend salvation from Christ's sacrifice to the Jews, who he says would be saved were they to believe - to have faith
So he is also mentioning justification by faith!