4 No doubt, if any one is unwilling to follow the Gospel itself, it is in his power [to reject it], but it is not expedient. For it is in man's power to disobey God, and to forfeit what is good; but [such conduct] brings no small amount of injury and mischief. And on this account Paul says, “All things are lawful to me, but all things are not expedient;” referring both to the liberty of man, in which respect “all things are lawful,” God exercising no compulsion in regard to him; and [by the expression] “not expedient” pointing out that we “should not use our liberty as a cloak of maliciousness,” for this is not expedient.
And again he says, “Speak every man truth with his neighbour.” And, “Let no corrupt communication proceed out of your mouth, neither filthiness, nor foolish talking, nor scurrility, which are not convenient, but rather giving of thanks.” And, “For you were sometimes darkness, but now are you light in the Lord; walk honestly as children of the light, not in rioting and drunkenness, not in chambering and wantonness, not in anger and jealousy. And such were some of you; but you have been washed, but you have been sanctified in the name of our Lord.” If then it were not in our power to do or not to do these things, what reason had the apostle, and much more the Lord Himself, to give us counsel to do some things, and to abstain from others?
But because man is possessed of free will from the beginning, and God is possessed of free will, in whose likeness man was created, advice is always given to him to keep fast the good, which thing is done by means of obedience to God.
Source: Adversus haereses (New Advent)