If Jesus is God...
1: Why does Jesus pray to himself?
2: Why does he say the father is greater than he in John 14:28?
3: Why does Christ distinguish between his will and Gods in Luke 22:42?
4: Why would Jesus not be all knowing then?
5: Why does it say there is one God and only one mediator between god and man that is Christ?
6: How could Jesus who is God be tempted in the wilderness if it is said God cannot be tempted?
7: Why does Jesus say he is the manifestation of God?
I'm gonna take these questions one by one. But here is some info you need to know beforehand. The authors of the Bible were not Trinitarian. WOAH DANIEL'S A HERETIC. No he's not. What he means is, Trinitarian theology was a systemization of Biblical truth. To say the Biblical authors were Trinitarian would be anachronistic. However, what they articulated gets us to Trinitarian theology. That is to say, Trinitarian thought was at the back of their minds the whole time, not the fore. So while they articulated things consistent with Trinitarian theology, they are often not worded the same way we would say them. Just like it is wrong to say Paul is a Calvinist, it is wrong to say that the Biblical writers were Trinitarian, even though they thought like one.
Jesus doesn't pray to himself. He prays to the father.
"When Jesus had spoken these words, he lifted up his eyes to heaven, and said, “Father, the hour has come; glorify your Son that the Son may glorify you, since you have given him authority over all flesh, to give eternal life to all whom you have given him. And this is eternal life, that they know you the only true God, and Jesus Christ whom you have sent. I glorified you on earth, having accomplished the work that you gave me to do. And now, Father, glorify me in your own presence with the glory that I had with you before the world existed." -John 17:1-5.
Jesus prays to the Father. And in doing so we get a glimpse of the Trinitarian relations in regards to the economy of salvation. "You have given him authority over all flesh," "having accomplished the work that you gave me to do," and statements like these show the humble submission of Christ to do the Father's will.
But one may ask, why did he have to pray to the Father if he was God? Good question. Because he was also man. Protestants often get wrapped up into making Christ seem so beyond us that we forget that "though he was in the form of God, did not count equality with God a thing to be grasped, but made himself nothing, taking the form of a servant, being born in the likeness of men" (Phil. 2:6). Jesus was a man too. The one who laid the foundations of the earth (John 1:3), was hungry. The source of eternal joy wept (John 11:35). And most of all, the one who the entire universe was created and was created for died (Col. 1:16; John 19:30). We often get so wrapped up in the deity of Christ that we forget his humanity. Jesus could be happy, sad, angry, hurt, surprised, tempted, tried, tired, hungry, and all of those things humans can be. Why? Because he was human! We have to get that. Yeah he walked on water, but he had to bathe like us. Yeah he fed the five thousand by multiplying the bread and fish, but he had to eat some too. If we understand the humanity of Christ, we may understand why he did some things that he did. Praying is one of those.
"You heard me say to you, 'I am going away, and I will come to you.' If you loved me, you would have rejoiced, because I am going to the Father, for the Father is greater than I." -John 14:28.
Does this verse automatically mean that Father in every way is greater than the Son? No. If I said, "I'm going to the White House to meet the President because the President is greater than I," you would probably be happy for me. That was Jesus' point. He was going to meet with someone great. But just because I said the President is greater than I am doesn't mean the President is greater than I am in being or essence. That is to say, the President is not a greater human being than I am. We are both equally human. But in rank/power/authority, the President is much greater than I am. So it was in this passage. There are many other verses in the same book that show Jesus' equality with God in being and essence (John 1:1; 10:30; and 20:28 just to name a few).
"... saying, 'Father, if you are willing, remove this cup from me. Nevertheless, not my will but yours, be done.'" -Lk. 22:42
Again, we're faced with the glaring reality of Christ's humanity. He's afraid. He doesn't want to suffer. He doesn't want to drink the wrath of God. BUT (and praise him for it!) he voluntarily and obediently, perhaps even conscientiously submitted his will to that of the Father's in order that "he might be just and the justifier of the one who has faith in Jesus" (Rom. 3:26). John 10:18 lays out that clearly Jesus laid down his life willingly. That doesn't mean that it's something he enjoyed. His humanity was scared. His humanity could be hurt and mangled and striped and torn. He was to experience the full wrath of God on the cross. He knew it. And why would he want that? Who would? No one! But Jesus was the one "who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God" (Hb. 12:2). Jesus endured it because he knew that it would mean the salvation of his people (which is the "for the joy" part).
Again, this is understanding the part of Christ's humanity.
In addition, Gregory of Nazianzus (329-390) said this in the midst of a debate about the nature of Christ: "That which is not assumed is not healed." That means that if Jesus did not take upon himself both a human body and a human will, how could our broken, sinful, bound, totally depraved will be changed? How could our will be healed? If Christ didn't take on a human body, could we have been saved? This is the point, when John says, "Καὶ ὁ λόγος σὰρξ ἐγένετο καὶ ἐσκήνωσεν ἐν ἡμῖν," (that is, "and the word became flesh and dwelt among us") he meant it. When the Word became flesh, it means he took all that it means to be human and became that- a human will and all.
"Now we know that you know all things and do not need anyone to question you; this is why we believe that you came from God." -John 16:30.
It seems to me Jesus does know all things. As part of his divine nature. But as part of his human nature, he is limited- by hunger, by thirst, by strength, and even by knowledge. In Matthew 24:36 we read that no one, not even the Son knows the day nor the hour of when the Son is to return; instead, it will come like a thief in the night. All we have here is the Biblical text. We can philosophize and theologize as much as we want, but at the end of the day, we have verses saying both that his knowledge in some sense is unlimited (John 16:30) and that his knowledge is in some sense limited (Matt. 24:36). This can be confusing as heck, to be honest. But that's what we're left with in the text. In the same way we understand that Christ was entirely divine and entirely human. That sounds like 200% not 100%. Confusing, but it's what the Bible teaches.
"This is good, and it is pleasing in the sight of God our Savior, who desires all people to be saved and to come to the knowledge of the truth. For there is one God, and there is one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all, which is the testimony given at the proper time." -1 Tim. 2:3-6.
God desires all people to be saved, and so made a way for them to be saved. Back in Genesis 3:15, we already have a promise of salvation. God accomplished this through his Son, who became flesh (read "the man") and died so that whoever believed may have everlasting life (John 1:14; 3:16; Tim. 2:6). But the reason Christ is the one mediator is because God offers a way of salvation. Jesus said he was the way, the truth, and the life, and that nobody can come to the Father except by him. This was because God appointed him to do this (Hb. 1:2). And like we said earlier, Jesus willingly accepted.
Perhaps one might wonder, if he is a mediator then doesn't that mean he is less than God the Father in some way? No, it doesn't. Here's why. Jesus was the one who became human. Jesus is the way to the Father. Jesus, not the Father, died on the cross. Jesus took our punishment on the cross by being a propitiation by his blood (Rom. 3:21-26). Jesus is the means of our salvation and that by which we are reconciled to God (the Father). Again, the Bible doesn't speak in Trinitarian terms, because it was at the back, not the fore, of their minds. They didn't have the problems we do today... or even that we had way back in the second and third centuries with Arianism and Docetism.
"Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil... Jesus said to him, "Again it is written, 'You shall not put the Lord your God to the test'"" (Matt 4:1, 7).
According to A Greek-English Lexicon of the New Testament and other Early Christian Literature (BDAG), πειράζω (the same word used for "tempted" in verse 1 and "put to the test" in verse 7) can mean "to make effort to do something; to endeavor to discover the nature or character of something by testing." I think this is of tantamount importance.
So I think that in Matthew, when the devil puts our Lord to the test, he is endeavoring "to discover the nature or character of [Jesus] by testing" him. We discover Christ's character when he is put to the test- the character of God. At the same time we see the Devil trying to put Christ to the test. Just like the Israelites sinned by putting God to the test at Massah (Ex. 17:7), the Devil sinned by putting Jesus to the test in the Wilderness. But unlike God at Massah, he did not do a miracle there. Instead he relied on the Holy Spirit and the Word of God to satisfy him, unlike Israel.
One more thing to notice. The text says you "shall not put the Lord your God to the test," not you "cannot put the Lord your God to the test."
"He was foreknown before the foundation of the world but was made manifest in the last times for the sake of you who through him are believers in God, who raised him from the dead and gave him glory, so that your faith and hope are in God." -1 Peter 1:20.
The answer to the question, "Why does Jesus say he is the manifestation of God?" is, simply, "He doesn't." Especially in the way most people understand the term "manifestation." Peter says he was made manifest. But being made manifest and being a manifestation of something are COMPLETELY different things. Manifestation can mean a form that something takes on. Rather Jesus was God made manifest (1 Tim. 3:16). As defined by Webster, manifest here means something that is able to be seen. Through Christ we can see God, because God has been made manifest!
Again, the greek word for to manifest is φανερόω, which BDAG defines as "to cause to become visible, reveal, expose publicly." God has exposed himself publicly through the revelation of God in Jesus Christ.
[Excursus: just because Manifestation and Manifest have the same root and lexeme does not mean that they necessarily convey the same meaning. He was manifested, made visible and revealed. He was not merely a form something had. He was the thing itself. "The Word [himself] became [was made manifest in the] flesh and dwelt among us."]
A long time ago, in only the second century, there was a dude named Origen. He was pretty weird, but very smart. He got a lot wrong, but he got a lot right. One of the things he got right was the doctrine of Eternal Generation. This is the argument he utilized, mind you, only about a generation (maybe two) after the apostles: "An eternal personal act of the Father, wherein, by necessity of nature, not by choice of will, He generates the person (not the essence) of the Son, by communicating to Him the whole indivisible substance of the Godhead, without division, alienation, or change, so that the Son is the express image of His Father's person, and eternally continues, not from the Father, but in the Father, and the Father in the Son."
Most basically what Origen says here is that for the Father to have eternally been the Father, he must have eternally begotten a Son. Begotten does not mean created. It means finds it's source from. So, the Son eternally finds his source from/in the Father of necessity because the Father is a father. You guys get this? It's like a guy can't be a husband without a wife. In the same way, God can't be Father without a Son- thus, Eternal Generation.
To understand the Spirit's place in intra-Trinitarian relations, I refer you to Thomas Aquinas in his Summa Theologica that is probably on CCEL, or St. Basil the Great's De Spiritu Sancto. They're too long for me to put up here right now, but just know that Christians have been defending the great doctrine of the triune God (the term Trinity only came with Tertullian, though) since the very beginning.
Hopefully some of these helped. But it's only by the grace of God that we come to true understanding because it is the Holy Spirit who leads us unto all truth.