Jesus says to his disciples in John, “I tell you the truth, anyone who has faith in me will do what I have been doing. He [or she] will do even greater things than these, because I am going to the Father.”
Of course our first reaction is to ask, “Greater than Jesus? How is that even possible?”
One possible meaning: Because I am going to the Father–because I am going to die and rise again–sin and death will be defeated and you will have even more power than you do now.
The Kingdom would be even more fully ushered in at the end of John. Is this what Jesus means?
This, at least, is what D.A. Carson suggests:
In short, the works that the disciples perform after the resurrection are greater than those done by Jesus before his death insofar as the former belong to an age of clarity and power introduced by Jesus’ sacrifice and exaltation. Both Jesus’ words and his deeds were somewhat veiled during the days of his flesh; even his closest followers, as the foregoing verses make clear, grasped only part of what he was saying. But Jesus is about to return to his Father, he is about to be glorified, and in the wake of his glorification his followers will know and make known all that Jesus is and does, and their every deed and word will belong to the new eschatological age that will then have dawned.
I think it could also be helpful to understand Jesus’ statement in light of the signs he has performed.
Jesus says the above in John 14, shortly after the conclusion of the “Book of Signs” portion of John–the first 12 chapters containing his “7 Signs.” John 13-21, then, constitute what scholars call the “Book of Glory.”
A sign, after all, is that which (while good in itself) points away from itself and to a greater, deeper, fuller reality. So if Jesus is referring to “greater things than these signs,” that is not so hard to grasp if we consider that signs always point to something greater anyway. One could read Jesus’ statement as a sort of tautology, where the “greater things” mean that somehow the deeper reality Jesus’ signs point to is more fully unearthed through the ministry of the disciples.
In other words, Jesus says to Philip, these signs are just a foretaste of the Kingdom of Heaven, and you and all the disciples after you are going to work and work and work together to keep bringing the Kingdom in.
When puzzling over John, I can think of no better place to turn than to Raymond E. Brown, to whom I give the last word. Note especially the final sentence of this paragraph.