For I will not venture to speak of anything except what Christ has accomplished through me to bring the Gentiles to obedience—by word and deed...
Romans 15:18 ESV

Paul is saying that he would take no credit for the sanctification of the mixed multitude that God was calling out of the nations.

Obedience to the Law of God is a primary goal of evangelism. It isn’t enough to say “I believe” if your behavior doesn’t change. Belief without obedience is a lie. Paul didn’t reach the nations of his day through mere speech, but through walking out the word. He obeyed the Law throughout his life. When he “became as a Greek” to the Greek, he didn’t mean that he sacrificed at their temples, kept their holy days, and ate unclean animals, but that he met them where they were, speaking to them in words and concepts they could understand and humbling himself to walk among them and fellowship with them.