3 As to the question whether lying is in any case justifiable and expedient, it has appeared to you that it ought to be solved by the example of our Lord's saying, concerning the day and hour of the end of the world, “Neither does the Son know it.” When I read this, I was charmed with it as an effort of your ingenuity; but I am by no means of opinion that a figurative mode of expression can be rightly termed a falsehood. For it is no falsehood to call a day joyous because it renders men joyous, or a lupine harsh because by its bitter flavour it imparts harshness to the countenance of him who tastes it, or to say that God knows something when He makes man know it (an instance quoted by yourself in these words of God to Abraham, “Now I know that you fear God”). These are by no means false statements, as you yourself readily see.
Accordingly, when the blessed Hilary explained this obscure statement of the Lord, by means of this obscure kind of figurative language, saying that we ought to understand Christ to affirm in these words that He knew not that day with no other meaning than that He, by concealing it, caused others not to know it, he did not by this explanation of the statement apologize for it as an excusable falsehood, but he showed that it was not a falsehood, as is proved by comparing it not only with these common figures of speech, but also with the metaphor, a mode of expression very familiar to all in daily conversation.
For who will charge the man who says that harvest fields wave and children bloom with speaking falsely, because he sees not in these things the waves and the flowers to which these words are literally applied?
Source: Letters (New Advent)