Paul condemns breaking Mosaic laws from which Christians are supposedly free

PAUL’S FIRST EPISTLE TO THE CORINTHIANS

1 Corinthians 5:1, Paul reproaches his followers that "there be fornication among them, and that one of them had committed incest by marrying his father's wife."

If Paul, as all his writings indicate, considered the Christians dispensed from observing the Mosaic Law, where was their boundary of religious duties or transgressions, seeing that Jesus had not promulgated a new code of laws? Surely, no permission had been granted by the founder of Christianity, so that his followers should observe part of the Mosaic laws, and reject the remainder.