4 Attend now to a wider and more difficult question. “And greater works than these,” says He, “will He show Him, that you may marvel.” “Greater than these.” Greater than which? The answer readily occurs: than the cures of bodily diseases which you have just heard: For the whole occasion of this discourse arose about the man who was thirty and eight years in infirmity, and was healed by the word of Christ; and in respect of this cure, the Lord could say, “Greater works than these He will show Him, that you may marvel.”
For there are greater, and the Father will show them to the Son. It is not “has shown,” as of a thing past, but “will show,” of a thing future; or, is about to show. Again a difficult question arises: Why, then, is there something with the Father that has not yet been shown to the Son? Is there something with the Father that was still hid from the Son when He spoke these words? For surely, if it be “will show,” that is to say, “is about to show,” then He has not yet shown; and He is about to show to the Son at the same time as to these persons, since it follows, “that you may marvel.” And this is a thing hard to see, how the Eternal Father does show something, as it were in time, to the coeternal Son, who knows all things that are with the Father.
Source: Tractates on the Gospel of John (New Advent)