# 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 most important" 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.