In the Gospels, on the night before His crucifixion, Jesus is recorded praying to God, and asking Him to somehow provide a way to escape the impending crucifixion, if it is possible (Matthew 26:39, Mark 14:36, Luke 22:42):
Matthew 26:39 (LEB)
39 And going forward a little he fell down on his face, praying and saying, "My Father, if it is possible, let this cup pass from me. Nevertheless, not as I will, but as you will."
Consider the above words of the Lord carefully, and whether one who is Almighty God could make such a prayer:
Firstly, Jesus prefaces His request with "if it is possible", which likely means "if it is in accordance with your will", as that is how the account in Luke renders it (Luke 22:42), and taking it in that sense fits the context. He then asks for something which ultimately God does not grant - to avoid the impending crucifixion.
The words that He uses in His prayer express that He was uncertain as to the precise will of God, and thought it was at least possible that God would allow Him to escape the crucifixion at that time, and so He asked for that to be granted. The fact that His request was ultimately not the will of God demonstrates that Jesus does not have all knowledge. He asked for something, seemed to think it was possible that God would grant it, and yet, it was not granted. If He knew beforehand that God did not wish to grant it, He would have been asking for something that He knew was against the will of God, and it could not in that case really be called a sincere request. Therefore, Jesus did not have the mind of God, which knows all things, and this means that He is a distinct Being from God.
In response, some may suggest that Jesus, although He knew this prayer would be denied, and was not in the will of God, merely spoke this as a demonstration that He would submit to the Father's will. But, this does not take into account the fact that He actually made a request. If this prayer were simply to demonstrate that He would submit to the Father's will, there would be no need to ask for a way to escape the fate of the cross, or express uncertainty as to whether it would be provided. Instead, He would merely need to say what He said at the end of the verse - something like, "I do not wish to go to the cross, yet not as I will, but as you will". His asking to escape cannot simply be ignored. He asked to escape because He wanted that prayer to be granted.
Secondly, the fact that Jesus desired this petition to be granted, and yet it was not granted, demonstrates that His will and God's will are distinct. Jesus wanted something, God wanted something else, and Jesus submitted His will, in faith, to God's. If God is a single, perfect, Being, can He have different, opposing wills?
In response, those who believe in the Deity of Christ may say, "He spoke from His human nature". The verse, however, gives no hint that He is not speaking as a single Person - Jesus Christ. There is only one Person speaking in the verse in question - a single subject, entreating another subject. Neither the Gospel writers, nor Jesus Himself, ever concern themselves with explaining how in one passage, Jesus spoke from His divine nature, saying something that was not true of His human nature, and in another passage, spoke from His human nature, saying something that was not true of His divine nature. These things are only read into various passages because those passages, if they are not qualified in such a way, create interpretive problems for the doctrine of the Deity of Christ. Yet, the Gospel authors never make such qualifications. There is no contextual reason to believe that Jesus is praying in His human nature for something to happen, which He knows in His divine nature cannot happen, and will never be granted.
Lastly, notice that under the Trinitarian reading, there actually is no meaningful submission taking place. The single Person speaking, Jesus, says "not as I will, but as you will". But, in reality, if He has the divine will as well, "not as I will, but as you will" becomes, "not as I will (in one part of myself), but as (I will in another part of myself)". If Jesus has the will of God, then the Person Jesus did not submit Himself to any other Being, and what appears to be a humble prayer of submission is actually just an extraordinarily complex metaphysical lesson disguised as a prayer.
When this passage is taken simply, and straightforwardly, it demonstrates that Jesus is not God, but rather one who submitted His will to God, and then was glorified by God on account of that, being the ultimate example of faith for believers to follow.