Most Christians didn't believe in the Trinity
An interesting statement I came across by Tertullian, writing ~220AD in defense of his Trinitarian view of God:
"The simple people (I will not call them unwise and unlearned) who always constitute the majority of believers are startled at the idea of Three-in-One. This is because the very basis of their faith is that polytheism is false and that there is only one true God. They don't understand that though he is the only God, he must be understood as Trinity. They think speaking of Trinity is a denial of the Oneness of God... so they accuse us of preaching two Gods or three Gods. Whereas they claim strongly that they worship only one God... in this way both Greeks and Latins affirm the oneness of God and deny the Trinity." (Tertullain, Against Praxeas, 3)
In short, the majority of Christians at the time were apparently non-Trinitarian and thought that such teaching contradicted their Jewish-derived belief in there being only one God.