The first reason for this is that knowledge about God and therefore acknowledgment of God is not possible without revelation; and knowledge of the Lord and therefore acknowledgment that all the fullness of divinity dwells physically in him is not possible without the Word, which is a garland of revelations. From the revelation they have been given, people are able to meet God, receive an inflow, and thus be made spiritual instead of earthly.
Early revelation spread throughout the whole world, and the earthly self distorted it in many different ways, giving rise to divergences, disagreements, heresies, and schisms among religions.
The second reason [for the diversity of opinions on God] is that the earthly self cannot comprehend anything about God; it can comprehend only the world, and conform it to itself. This is why it is among the axioms of the Christian church that the earthly self is against the spiritual self, and that they battle each other. People then have come to acknowledge from the Word [or] from some other revelation that there is a God, and yet in both the past and the present they have had a diversity of opinions on the nature and the oneness of God.
from True Christianity, Volume 1, Page 12