If Jesus is God, why did He say "The Father is greater than I?"
Observe the context: "Ye have heard how I said unto you, I go away, and come again unto you. If ye loved me, ye would rejoice, because I said, I go unto the Father: for my Father is greater than I" (John 14:28).
To understand what this means, we need a proper understanding of the incarnation. The best passage to describe the incarnation is Philippians 2:6-8, which says "who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross."
Now, just verse 6 ought to be enough to settle the dispute about whether or not Jesus is God. Jesus is not just in the form of God, but has equality with God, something which is sheer blasphemy if Jesus is not, Himself, God. There were no gods before God, nor shall there be any god after God (Isaiah 43:10), and God will not share His glory with anything or anyone (Isaiah 42:8). Therefore, in order to be equal to God, Jesus must be God.
But Jesus humbled Himself by taking on the form of a man. There are even several indicators of His divine nature here in that it says Jesus did these things Himself. He made Himself of no reputation, and became obedient to death. Who can resist death but God? Even Satan is going to die eventually, but Jesus became obedient to death.
This humbling of Himself is what makes the Father greater than Jesus. The incarnation emptied Jesus of several of His divine attributes; His physical immortality, His omnipotence, even His omniscience (Matthew 24:36; Mark 13:32).
When anti-Trinitarians cite John 14:28, they not only ignore the incarnation, but even the entire verse itself. Jesus didn't just say "the Father is greater than I", but rather "If ye loved me, ye would rejoice, because I said, I go unto the Father: for my Father is greater than I." Note, if the disciples loved Jesus, they would rejoice that He's leaving. Jesus is getting some benefit out of this. Specifically, He is gaining "the glory which I had with thee before the world was." (John 17:5). Note that this verse again affirms that Jesus shared His glory with God, as well as Jesus' existence prior to the creation of this world.
Rather than being some attempt to bash the doctrine of the Trinity, John 14:28, in its correct context, is very powerful evidence in favor of the Trinity. Anyone who claims otherwise is accusing the New Testament, including Jesus' own quotations, of outright blasphemy. For the pseudo-Christian cults that usually use this argument against the Trinity, that's not a safe place to stand, intellectually or morally.