8 Wherefore, most dearly beloved, whosoever says, My will suffices for me to perform good works, declines to the right. But, on the other hand, they who think that a good way of life should be forsaken, when they hear God's grace so preached as to lead to the supposition and belief that it of itself makes men's wills from evil to good, and it even of itself keeps them what it has made them; and who, as the result of this opinion, go on to say, "Let us do evil that good may come," --these persons decline to the left.
This is the reason why he said to you, "Turn not aside to the right hand, nor to the left;" in other words, do not uphold free will in such wise as to attribute good works to it without the grace of God, nor so defend and maintain grace as if, by reason of it, you may love evil works in security and safety,--which may God's grace itself avert from you! Now it was the words of such as these which the apostle had in view when he said, "What shall we say, then? Shall we continue in sin that grace may abound?"
And to this cavil of erring men, who know nothing about the grace of God, he returned such an answer as he ought in these words: "God forbid. How shall we, that are dead to sin, live any longer therein?" Nothing could have been said more succinctly, and yet to the point. For what more useful gift does the grace of God confer upon us, in this present evil world, than our dying unto sin? Hence he shows himself ungrateful to grace itself who chooses to live in sin by reason of that whereby we die unto sin.
May God, however, who is rich in mercy, grant you both to think soundly and wisely, and to continue perseveringly and progressively to the end in every good determination and purpose. For yourselves, for us, for all who love you, and for those who hate you, pray that this gift may be attained,--pray earnestly and vigilantly in brotherly peace. Live unto God. If I deserve any favour at your hands, let brother Florus come to me.
Source: Letters (New Advent)