32 But, as for that which is written, that God did good to the Hebrew midwives, and to Rahab the harlot of Jericho, this was not because they lied, but because they were merciful to God's people. That therefore which was rewarded in them was, not their deceit, but their benevolence; benignity of mind, not iniquity of lying. For, as it would not be marvellous and absurd if God on account of good works after done by them should be willing to forgive some evil works at another time before committed, so it is not to be marvelled at that God beholding at one time, in one cause, both these, that is, the thing done of mercy and the thing done of deceit, did both reward the good, and for the sake of this good forgive that evil.
For if sins which are done of carnal concupiscence, not of mercy, are for the sake of after works of mercy remitted, why are not those through merit of mercy remitted which of mercy itself are committed? For more grievous is a sin which with purpose of hurting, than that which with purpose of helping, is wrought. And consequently if that is blotted out by a work of mercy thereafter following, why is this, which is less heinous, not blotted out by the mercy itself of the man, both going before that he may sin, and going along with him while he sins?
So indeed it may seem: but in truth it is one thing to say, “I ought not to have sinned, but I will do works of mercy whereby I may blot out the sin which I did before;” and another to say, “I ought to sin, because I cannot else show mercy.” It is, I say, one thing to say, “Because we have already sinned, let us do good,” and another to say, “Let us sin, that we may do good.” There it is said, “Let us do good, because we have done evil;” but here, “Let us do evil that good may come.” And, consequently, there we have to drain off the sink of sin, here to beware of a doctrine which teaches to sin.
Source: To Consentius: Against Lying (New Advent)