John Calvin Commentary 1 Corinthians 9:21

"to them that are without law, as without law, not being without law to God, but under law to Christ, that I might gain them that are without law." — 1 Corinthians 9:21 (ASV)

Though not without law to God. He wished by this parenthetical remark to soften the harshness of the expression, for it might have seemed harsh at first glance for it to be said that he had come to be without law. Therefore, so that this might not be misunderstood, he had added, by way of correction, that he had always kept one law in view—that of subjection to Christ.

By this too, he hints that hatred was stirred up against him groundlessly and unreasonably, as if he called people to unbridled licentiousness while he taught exemption from the bondage of the Mosaic law. Now he expressly calls it the law of Christ to wipe away the groundless reproach with which the false apostles branded the gospel, for he means that in the doctrine of Christ, nothing is omitted that might serve to give us a perfect rule of upright living.