Unitarian Questions: Is The Trinity a mere invention of men?

I was recently challenged with this argument:

God is not shy to say hes God, he has always boldly and authoritatively stated that throughout the old testament. Trinity was invented 300 years after Jesus died that should be proof enough of a false doctrine. Abraham fell on his face and prayed to God, so did David and so did Jesus. 1 Corinthians 15:27-28 - For he “has put everything under his feet.” Now when it says that “everything” has been put under him, it is clear that this does not include God himself, who put everything under Christ. 28 When he has done this, then the Son himself will be made subject to him who put everything under him, so that God may be all in all. So as you can see it clearly says that God does not put himself under himself, so if Christ is put under God then he cannot be God, simple and case closed. Go read more thank you. Your knowledge betrays you.

The first point to discuss is about history. Was the trinity indeed invented 300 years after Jesus died? Historical facts tells us that the vision of one God as being with multiple persons predate Christianity. It was not invented 300 years after Jesus. Historical evidence does not support this argument. For details please read this article: Jtruthtobelieve.blogspot.com/2015/03/the-deity-of-messiah.html?m=1 In this article it's explained that:

1. The Targum of Johnathan Ben Uzziel from the scoop of Hilel (1st Century BCE) says Messiah is God and everlasting.
2. Judaism in Alexandria at the time of Jesus (Philo, before 50 CE) wrote similar views as those exposed in John 1: The Word is God, though is not the same as the person of God who is Above his creation.
3. Church fathers writings before year 107 CE called Jesus as God
4. Other Targums written between 1st and 3rd Century call the Messiah by the name of YHWH and say that the word created the world
5. Talmud, Midrash and Mishnah writings also speak of Messiah being the Spirit of God, named YHWH, being eternal

My answer was responded in the following manner:

Yes it was invented and it was by turtullion. Such a pagan faith. God in multiple persons how could you even say that and not be worried? From old testament to new the oneness of God is stressed? Moses worshiped God, David, Abraham, Jesus too. There is not a single verse in the bible where it says God exists in three personalities NOT A SINGLE. Infact history actually does not support your concept of trinity. God has only sent people on his behalf to represent him, he has never and never will come down in human form never. Thats what the bible teaches us that Jesus was sent by God. If God existed in three personalities then their will would be one? even an ignorant human being like you knows that Jesus will was different from God but he came down from heaven not to do his will but the will of him that sent Jesus. One God in tree personalities = 1 will. Your concept is so disgustingly flawed im sorry it really is. You cant tell me im wrong for worshipping the father just like the bible instructs and just as Jesus instructs? But i can tell you your undeniably wrong for saying a man is God and that there are 3 forms of God. Im doing what God, Jesus and the bible tells me to do and that is Worship the father because hes the "only true God". Im doing the right thing. YOU are not and you know that but still you will blabber on its ok.

With regards to Tertullian, he died in 225 CE, that's less than two hundred years after Christ. Also, he was not the inventor of the Trinity. He is the author of the oldest writting available to us today which uses the convenient term of "Trinitas" to compile the group of truths expressed in the Bible about the natures and essence of the Creator. That doesn't make him the inventor of the Trinity, because older authors coincede with his theological views of God, without using the "Trinitas" term.

Another issue brough forward is wether God could enter his creation in human form. As you may note, there was no verse in the Bible used to back up this statement. The reason is because no verse in the Bible closes the door to this possibility.