A.T. Robertson Commentary Romans 6:15

"What then? shall we sin, because we are not under law, but under grace? God forbid." — Romans 6:15 (ASV)

What then? (τ ουν?). Another turn in the argument about the excess of grace.

Shall we sin? (αμαρτεσωμεν?). First aorist active deliberative subjunctive of αμαρτανω. "Shall we commit sin" (occasional acts of sin as opposed to the life of sin as raised by επιμενωμεν τη αμαρτια in verse 1)?

Because (οτ). The same reason as in verse 1 and taken up from the very words in verse 14. Surely, the objector says, we may take a night off now and then and sin a little bit "since we are under grace."