A brief outline-
12-14: Paul’s personal testimony about how grace changed him
15-16: Paul’s summary of God’s mercy in the gospel
17: Paul’s doxology in response to these two things
Some struggle with the flow of thought from verse 11. Verse 11 says, “…the glorious gospel of the blessed God, with which I have been entrusted.” And that last phrase seems to remind Paul of how undeserving of this ministry he is. So he talks about how he used to be a persecutor of the Church and how grace changed him. And two quick notes about the “chief of sinners” phrase: some look at this passage and think we are all supposed to say this about ourselves, namely, that ‘I am the chief of sinners.’ But Paul’s teaching is not that we are all the foremost of sinners, but rather, that he actually was the foremost of sinners. He actually killed Christians! And if God can save the foremost of sinners, he can save anyone.
Secondly, why does he say, present tense, “I am the chief of sinners”? Does he still sin just as he did before he was converted? May it never be! He is trying to convey God’s mercy in the gospel, meaning, none of us are deserving of eternal life– especially Paul, who used to persecute Jesus! So Christians should not call themselves “sinners” in the same way they used to be “sinners” before conversion, but we should call ourselves “sinners” in the sense that a million years from now, we will be just as undeserving of salvation as the day God gave us new hearts.