en_tn/1ti/01/15.md

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.