42 For the Lord had said in the gospels, Work not for the meat which perishes, but for the meat which abides unto life eternal, which the Son of Man shall give unto you: for Him the Father, even God, has sealed. They said therefore unto Him, What must we do that we may work the works of God? And He said unto them, This is the work of God, that you believe in Him Whom He has sent. In setting forth the mystery of His Incarnation and His Godhead our Lord has also uttered the teaching of our faith and hope that we should work for food, not that which perishes but that which abides for ever; that we should remember that this food of eternity is given us by the Son of Man; that we should know the Son of Man as sealed by God the Father; that we should know that this is the work of God, even faith in Him Whom He has sent.
And Who is it Whom the Father has sent? Even He Whom the Father has sealed. And Who is He Whom the Father has sealed? In truth, the Son of Man, even He who gives the food of eternal life. And further who are they to whom He gives it? They who shall work for the food that does not perish. Thus, then, the work for this food is at the same time the work of God, namely, to believe in Him Whom He has sent. But these words are uttered by the Son of Man. And how shall the Son of Man give the food of life eternal? Why, he knows not the mystery of his own salvation, who knows not that the Son of Man, bestowing food unto life eternal, has been sealed by God the Father. At this point I now ask in what sense are we to understand that the Son of Man has been sealed by God the Father?
Source: On the Trinity (New Advent)