I used to be Trinitarian. It never really made sense to me, but it was all I knew. I had never heard of the Oneness view until I met an Apostolic friend. I have nothing against Trinitarians. I used to be one of them. I think Oneness Christians and Trinitarian Christians have the same God. Both are equally Christian. We just have different perspectives of God.
These are the reasons I no longer agree with the doctrine of the Trinity.
I see one God in the bible.
So where does the confusion come in?
Man tries to explain how God can be in heaven as the father, in the flesh, in the spirit form, and everywhere all at the same time. Here's the thing - God is omnipresent. He is present everywhere all the time. He is able to manifest himself in multiple ways. He is not divided or weakened. The doctrine of the Trinity attempts to use man's logic to explain how one God can be the father and the son and the holy spirit. The trinitarian belief holds that God is one god but 3 distinct persons without actually being separate persons. So the father is the son, the son is the father, the spirit is the father and the son, etc... but they are distinct from each other. That really doesn't make sense. The bible says God is one. It never says God is 3 in 1 or that there are 3 gods.
Oneness, on the other hand, believes that God is one. Period. Just one God. Just like God tells us in his word (the bible) that he is one. We Oneness believers feel that the doctrine of the Trinity is man-made and does not line up with scripture. We believe that God can be in the mode of Father, Son, and Holy Spirit all while maintaining full deity. We believe that God is omnipresent, omnipotent, and capable of doing everything he says he does even if we can't understand it by our human logic. A human man can be someone's son, someone's husband, and someone's father without being 3 different people. Those are all roles that one person fulfills. If we, as mere humans, can be one person that is someone's child, someone's spouse, and someone's parent, would God not be capable of being the Father, Son, and Holy Spirit without dividing the person?
I personally feel that the Trinitarian explanation of God is not biblical. The Oneness doctrine is based on the bible's explanation of who God is. The challenge is understanding the mystery of God and how Jesus robed himself in flesh born fully human yet remained fully God. It is hard to grasp, but we mustn't use our human understanding as 'logical' excuses to argue against scriptures that show oneness or to defend doctrines that go against the word of God.