What shall we say then? - This is a mode of presenting an objection. The objection refers to what the apostle had said in Romans 5:20. What shall we say to such a sentiment as that where sin abounded grace did much more abound?
Shall we continue in sin? ... - If sin has been the occasion of grace and favor, ought we not to continue in it, and commit as much as possible, in order that grace might abound? This objection the apostle proceeds to answer. He shows that the consequence does not follow; and proves that the doctrine of justification does not lead to it.