Why did Jesus pray?
The "Iglesia Ni Cristo" teaches that Jesus is not God. In teaching this, they often point to the prayers of Christ. They say that if he prayed to God, he must not be God or else he would be praying to himself. Is this a valid argument?
When we look at what the doctrine of the Trinity teaches, we see that this argument poses a strawman. When we look to what the scriptures say, we see that Jesus prayed in submission to the Father even though he is God.
Jesus does not pray to himself.
The first error the INC makes is assuming that God is like his creation, unipersonal - that there is only one person of God. They presuppose this without offering a proof from scripture. The doctrine of the Trinity teaches that God is tripersonal - that there are three persons of the one and only true God. Therefore, when an INC proponent accuses a Trinitarian of believing that Jesus prayed to himself, they are not arguing against the Trinity, but against a straw man. Trinitarians teach the Biblical doctrine that Jesus is a distinct person from his Father. Therefore, Trinitarians believe that when Jesus prayed to the Father, he was praying to another person, not to himself.
Jesus prays to the Father as God because the Father is God.
This point should be obvious, but Trinitarians also agree that the Father is God. Therefore, when Jesus addresses the Father as God, it is because he is God.
Jesus prays to the Father because he is a man.
Additionally, Jesus acted in willing submission to Father and he acted as a perfect man. Therefore, when he calls the Father his God, he is performing the duties of a man who must submit to God.
Jesus demonstrates that he is God in his high priestly prayer.
John 17:5 (ESV) - 5 And now, Father, glorify me in your own presence with the glory that I had with you before the world existed
When Jesus prayed, he prayed to one who shared his glory with him before the world existed. This is not a prayer that can be made by a mere man. It is a prayer that can only be made by someone who is God.
Trinitarians believe that Jesus is a distinct person from the Father, that the Father is God, and that Jesus is a man. Therefore, it makes perfect sense that Jesus would pray to the Father. This does not conflict with the Trinitarian belief that in addition to being a man, Jesus is also God. The INC argues against a strawman when they say that the Trinity is wrong because Jesus prayed to the Father. Rather, Jesus praying to the Father is entirely consistent and even evidence of the Trinity.
Additionally, the INC ignores the contents of Christ's prayers when they say that Jesus is mere man and not also God. He prayed prayers that only someone who is God and pre-existed the universe could possibly pray.