67 We can now understand why He said that He knew not the day. If we believe Him to have been really ignorant, we contradict the Apostle, who says, In Whom are all the treasures of wisdom and knowledge hidden. There is knowledge which is hidden in Him, and because it has to be hidden, it must sometimes for this purpose be professed as ignorance, for once declared, it will no longer be secret. In order, therefore, that the knowledge may remain hidden, He declares that He does not know.
But if He does not know, in order that the knowledge may remain hidden, this ignorance is not due to His nature, which is omniscient, for He is ignorant solely in order that it may be hidden. Nor is it hard to see why the knowledge of the day is hidden. He exhorts us to watch continually with unrelaxing faith, and withholds from us the security of certain knowledge, that our minds may be kept on the stretch by the uncertainty of suspense, and while they hasten towards and continually look for the day of His coming, may always watch in hope; and that, though we know the time must come, its very uncertainty may make us careful and vigilant.
Thus the Lord says, Therefore be ye also ready, for you know not what hour the Son of Man shall come; and again, Blessed is that servant whom His lord, when He comes, shall find so doing. The ignorance is, therefore, a means not to delude, but to encourage in perseverance. It is no loss to be denied a knowledge which it is an advantage not to have, for the security of knowledge might breed negligence of the faith, which now is concealed, while the uncertainty of expectation keeps us continually prepared, even as the master of the house, with the fear of loss before his eyes, watches and guards against the dreaded coming of the thief, who chooses the time of sleep for his work.
Source: On the Trinity (New Advent)