God's Messengers to Humanity have been called The Word of God, Prophets, and Manifestations.
The key cause of religious disunity is that people believe a particular Manifestation is "better" or "more God-like" or "stronger" or "more loving" or "________".
If we look closely at what these Manifestations have told us, we'll see Their Absolute Unity of Purpose:
"Be ye assured, moreover, that the works and acts of each and every one of these Manifestations of God, nay whatever pertaineth unto them, and whatsoever they may manifest in the future, are all ordained by God, and are a reflection of His Will and Purpose.
"Whoso maketh the slightest possible difference between their persons, their words, their messages, their acts and manners, hath indeed disbelieved in God, hath repudiated His signs, and betrayed the Cause of His Messengers."