17 But yet if the option were proposed to the man who chose to burn incense to idols rather than yield his body to abominable lust, that, if he wished to avoid that, he should violate the fame of Christ by some lie; he would be most mad to do it. I say more: that he would be mad, if, to avoid another man's lust, and not to have that done upon his person which he would suffer with no lust of his own, he should falsify Christ's Gospel with false praises of Christ; more eschewing that another man should corrupt his body, than himself to corrupt the doctrine of sanctification of souls and bodies.
Wherefore, from the doctrine of religion, and from those utterances universally, which are uttered on behalf of the doctrine of religion, in the teaching and learning of the same, all lies must be utterly kept aloof. Nor can any cause whatever be found, one should think, why a lie should be told in matters of this kind, when in this doctrine it is not right to tell a lie for the very purpose of bringing a person to it the more easily. For, once break or but slightly diminish the authority of truth, and all things will remain doubtful: which unless they be believed true, cannot be held as certain.
It is lawful then either to him that discourses, disputes, and preaches of things eternal, or to him that narrates or speaks of things temporal pertaining to edification of religion and piety, to conceal at fitting time whatever seems fit to be concealed: but to tell a lie is never lawful, therefore neither to conceal by telling a lie.
Source: On Lying (New Advent)