# This message is reliable

"This statement is true"

# worthy of all acceptance

"we should receive it without any doubt" or "deserves for us to accept it with full confidence"

# sinners, of whom I am chief

"sinners, of whom I am the chief sinner" or "sinners, of whom I am the worst." Paul is saying that he has sinned more than any other sinner, not that he has been the leader of a group of sinners.