Romans 6:17-18

¹⁷ But God be thanked that though you were slaves of sin, yet you obeyed from the heart that form of doctrine to which you were delivered.

¹⁸ And having been set free from sin, you became slaves of righteousness.

though you were (v.17) = emphasis is on the contrast between what they were (servants of sin) and what they now are (servants of righteousness).

you obeyed (v.17) = tense points back to the moment they believed — which accomplished permanent results

from the heart (v.17) = voluntary and earnest

form (v.17) = cast or mold — Believers are the molten material, gospel truth is the mold that shapes our character.

The change of masters (v.18) results from faith and is accomplished by God (not by believers themselves).