The Christian teaching of the Trinity is mysterious and cognitively challenging. The doctrine of the Trinity is that God is one God, eternally existent in three persons. That’s not tritheism, with three gods who work in harmony; neither is it unipersonalism, the notion that sometimes God takes one form and sometimes he takes another, but that these are simply different manifestations of one God. Instead, trinitarianism holds that there is one God in three persons who know and love one another. God is not more fundamentally one than he is three, and he is not more fundamentally three than he is one.
– Tim Keller