Probably the most fundamental difference between Mormon beliefs and other Christian Churches resides in this one topic. It’s the belief in the Christian version of “The Trinity”, and the Mormon belief in “The Godhead”.
Most of the differences between these two belief systems comes down to one question. Are the Father, the Son, and the Holy Ghost the same person/substance or different personages? The Trinity teaches that they are 3-in-1, meaning they are all the same person/substance. Along with this is that they are “incomprehensible” (why they call it “the Mystery of the Trinity”). It cannot be fully understood. The Mormon version of the Godhead teaches that they are separate and distinct personages. But they teach that you CAN understand and comprehend who they are (John 17:3 “And this is life eternal, that they might know thee, the only true God, and Jesus Christ whom thou hast sent“).
The mainstream Christian doctrine of the Trinity did not begin in the times of Jesus or the Apostles. It was formally written and accepted as dogma in the 4th century A.D. after the Council at Nicea under Emperor Constantine’s direction.
The Catholic Encyclopedia, and indeed Christian history itself indicate that this doctrine wasn’t formulated until this time. There is no canonical scripture that indicates that the Father, Son and Holy Ghost are of the same substance, or that they are “3 Gods in One” as the doctrine of the Trinity indicates. In fact, almost every scripture describes Jesus as “the Son of God”, and says even less about who the Holy Ghost is.
In contrast, the Mormon belief in The Godhead describes the Father, Son and Holy Ghost as “three separate and distinct personages” being “one in purpose” but not substance.
They teach that Jesus was the Son of the Eternal Father, just as he described himself in the Bible. And they believe the Holy Ghost is a personage of Spirit, who bears witness of truth. The 3 make up the Godhead.
The 1st “Article of Faith” of the Church of Jesus Christ of Latter-Day Saints (Mormon) says “We believe in God the Eternal Father, and in His Son Jesus Christ, and in the Holy Ghost”.