What does "chief of sinners" mean in the Bible?
The phrase means the leader or foremost of sinners, not necessarily the worst. In the context, it refers to Paul, whom God saved on the road to Damascus as a pattern for all who would afterward believe on Him unto life everlasting.
Les explains that Paul, by inspiration of the Holy Spirit, lays out that at the salvation experience on the road to Damascus, God saved the chief of sinners, not the worst necessarily, though Paul was apologetic for that throughout his letters, but the leader of sinners. Everyone now coming into the body of Christ is patterned after him and follows him. Les contrasts this with Peter at Pentecost, who said repent and be baptized for life everlasting, whereas Paul says believe the gospel. This is a pattern to them which should hereafter believe on Him to life everlasting, going from Paul onward, not retroactive.