5 “And greater works than these will He show Him, that you may marvel.” Here again we are embarrassed. And who is there that may worthily investigate this so great a secret? But now, in that He has deigned to speak to us, Himself opens it. For He would not speak what He would not have us understand; and as He has deigned to speak, without doubt He has excited attention: for does He forsake any whom He has roused to give attentive hearing? We have said that it is not in a temporal sense that the Son knows—that the knowledge of the Son is not one thing, and the Son Himself another; nor one thing His seeing, Himself another; but that the seeing itself is the Son, and the knowledge as well as the wisdom of the Father is the Son; and that that wisdom and seeing is eternal and co-eternal with Him from whom it is; that it is not something that varies by time, nor something produced that was not in being, nor something that vanishes away which did exist.
What is it, then, that time does in this case, that He should say, “Greater works than these He will show Him”? “He will show,” that is, “He is about to show.” Hath shown is a different thing from will show: has shown, we say of an act past; will show, of an act future. What shall we do here, then, brethren? Behold, He whom we had declared to be co-eternal with the Father, in whom nothing is varied by time, in whom is no moving through spaces either of moments or of places, of whom we had declared that He abides ever with the Father seeing, seeing the Father, and by seeing existing; He, I say, here again mentioning times to us, says, “He will show Him greater works than these.”
Is He then about to show something to the Son, which the Son does not as yet know? What, then, do we make of it? How do we understand this? Behold, our Lord Jesus Christ was above, is beneath. When was He above? When He said, “What things soever the Father does, these same also the Son does in like manner.” Whence know we that He is now beneath? Hence: “Greater works than these He will show Him.” O Lord Jesus Christ, our Saviour, Word of God, by which all things were made, what is the Father about to show You, that as yet You know not?
What of the Father is hid from You? What in the Father is hid from You, from whom the Father is not hid? What greater works is He about to show You? Or greater than what works are they which He is to show You? For when He said, “Greater than these,” we ought first to understand the works than which are they greater.
Source: Tractates on the Gospel of John (New Advent)