A.T. Robertson Commentary Romans 6:17

"But thanks be to God, that, whereas ye were servants of sin, ye became obedient from the heart to that form of teaching whereunto ye were delivered;" — Romans 6:17 (ASV)

Whereas ye were (ητε). Imperfect but no "whereas" in the Greek. Paul is not grateful that they were once slaves of sin, but only that, though they once were, they turned from that state.

To that form of doctrine whereunto ye were delivered (εις ον παρεδοθητε τυπον διδαχης). Incorporation of the antecedent (τυπον διδαχης) into the relative clause: "to which form of doctrine ye were delivered." See on 5:14 for τυπον. It is hardly proper to take "form" here to refer to Paul's gospel (2:16), possibly an allusion to the symbolism of baptism which was the outward sign of the separation.