Jamieson, Fausset and Brown on Romans 6:17

17. But God be thanked, that ye were the servants of Sin--that is, that this is a state of things now past and gone.

but ye have obeyed from the heart that form of doctrine which was delivered you--rather, "whereunto ye were delivered" (Margin), or cast, as in a mould. The idea is, that the teaching to which they had heartily yielded themselves had stamped its own impress upon them.