Trinity and Jesus' Omniscience

The issues you raise have to do not only with the Trinity, but also with the "hypostatic union" -- the doctrine that the one person Christ has both a human nature and a divine nature. He is fully God and fully man. In the incarnation, the eternal Son of God was born as a man, permanently uniting with a human body (John 1:14) and soul (Matt. 27:50; "soul" is synonymous with "spirit").

In the hypostatic union, Jesus' two natures are totally separate (like the persons of the Trinity are totally separate), but they are united in one person (like the persons of the Trinity are united in one essence). Because they are totally separate, each nature retains its own attributes. That means that in his human nature, Jesus' knowledge is limited to what he has learned as a man, while in his divine nature he is totally omniscient, knowing everything.

Like the doctrine of the Trinity, the doctrine of the hypostatic union is just a way to try to summarize everything the Bible says about a particular subject. In reading the Bible, it may seem that Jesus and the Father are totally separate beings. This impression develops in large part because the Bible usually speaks about Jesus from the perspective of his human nature, and about God from the perspective of the person of the Father. Since Jesus' human nature is not part of the Trinity, the Bible's language draws only division between the identities of Jesus and God the Father when it speaks from these perspectives.

Another difficulty is that the Bible doesn't start talking about Jesus until the New Testament, but the New Testament is a pretty late publication in the broad scheme of the Bible. For all of history prior to the New Testament, God had revealed himself as one God. When God hinted at ideas expressed in the doctrine of the Trinity, these hints were easily explained in some other fashion by those who read the Old Testament originally. Without question, the Old Testament stresses the fact that there is only one God.

When Jesus came along, however, he began to teach that he himself was equal with God (John 5:18) -- but he did not teach that he was totally identical with the God of the Old Testament. Rather, he spoke of God as his Father. Lest anyone think that he was a new or different God, Jesus affirmed that there was only one God (e.g. Mark 12:29-34; John 5:44; 17:3). How could Jesus be equal with God and not be God? Well, John explains in John 1:1 that Jesus (whom John calls "the Word"; compare John 1:14) was "in the beginning," that he was "with God," and that he "was God." John says both that the Word was "with God," showing that the Word is distinct from God, and that the Word "was God," showing that the Word was not distinct from God. It can be argued that the grammar of John 1:1 means "the Word was divine" rather than "the Word was God." While I don't agree with this assessment given the context that John had already introduced the Word "God" as the name of God in the immediately preceding phrase, in the long run it doesn't really matter which reading one adopts. If there is only one God, then to be divine is to be that one God. While this passage doesn't speak of the Holy Spirit, it does express ideas that are reconciled only by saying that there is only one God, and that in God there are multiple distinct "persons" (or whatever word we choose to identify the distinction) who each may be called in and of themselves "God." John comes back to this idea in John 1:18 by calling Jesus the "only begotten God." Adding the Holy Spirit to the equation gives us three, making the Trinity, but the problem of how these distinct "persons" exist together is raised most clearly in the discussion in John 1.

John 1:1-18 is also a great passage to demonstrate the hypostatic union. "The Word was God" (John 1:1), and "the Word became flesh and dwelt among men" (John 1:14). When the Word became flesh, he did not cease to be God, but added "flesh" to his being. We know from other passages that Jesus' "flesh" included a soul/spirit (Matt. 27:50; Mark 2:8; Luke 23:46; John 11:33; 13;21; 19:30). Jesus was both man (1 Tim. 2:5) and God (John 1:1; Tit. 2:13; 2 Pet. 1:1).

So, when Jesus expressed lack of knowledge, he was speaking as a man. "My God, my God, why have you forsaken me?" is a bit of a special case though. Even as a man Jesus knew why God had forsaken him -- God forsook Christ because Christ was stained with the guilt of our sin. Jesus knew that he was an atoning sacrifice, and knew this meant that God would forsake him. His cry was a rhetorical exclamation expressing his grief. It was also a reference to Psalm 22 -- he cried out the "title" of that psalm, making reference to the fact that he was fulfilling its prophecy. There are clearer cases of Jesus not knowing things (e.g. Matt. 24:36 // Mark 13:32 // Luke 10:22).