44 Therefore after this manifold and precise revelation of the presence of the Father's nature in Himself, He goes on to say, For Him has the Father sealed, even God. It is the nature of a seal to exhibit the whole form of the figure graven upon it, and that an impression taken from it reproduces it in every respect; and since it receives the whole of that which is impressed, it displays also in itself wholly whoever has been impressed upon it. Yet this comparison is not adequate to exemplify the Divine birth, because in seals there is a matter, difference of nature, and an act of impression, whereby the likeness of stronger natures is impressed upon things of a more yielding nature.
But the Only-begotten God, Who was also through the Mystery of our salvation the Son of Man, desiring to point out to us the likeness of His Father's proper nature in Himself, said that He was sealed by God; because the Son of Man was about to give the food of eternal life, and that we thereby might perceive in Him the power of giving food unto eternity, in that He possessed within Himself all the fullness of His Father's form, even of the God Who sealed Him: so that what God had sealed should display in itself none other than the form of the God Who sealed it. These things indeed the Lord spoke to the Jews, who could not receive His saying because of unbelief.
Source: On the Trinity (New Advent)