Basically, Mormonism started out to be extremely Trinitarian and then drifted away from this traditional belief toward something very radical. They are now slowly moving back to traditional Trinitarian beliefs so well explained in the Book of Mormon. (Actually, the Book of Mormon contains a very clear scriptural basis for the Trinitarian doctrine that is only alluded to in the Bible or supplemented by glosses.)
I was raised in a very conservative Mormon family, a family that has been Mormon since 1840.
This is what I was taught as a child and what I taught as a missionary:
God the Father was once a man like we are now and we may progress as he did and become a God like him.
We have always existed as intelligences. God the Father and one of his many wives created our Spirit bodies to house our intelligences. Jesus was the first born spirit offspring of God the Father (this is why Mormons call Jesus their “Elder Brother”). Satan was the second born spirit. The rest of us came along after these two spirits.
God the Father and his wives have physical bodies and in order to progress to be like God the Father we also need physical bodies. God devised a plan (the organization of the Earth from pre-existing and co-eternal matter) so we could receive bodies. Jesus supported this plan and Satan opposed it. Satan and his followers were caste out of heaven, 1/3 of all the spirit children of God the Father. Of the 2/3rds remaining in heaven, some of Spirit children of God had been valiant followers and supporters of Jesus in his fight against Satan while others had not been. Those that were not valiant were born as descendants of Cain who was marked by God with Black skin to protect him from being hunted down and killed after he had murdered his brother Able.
God the Father created all things by organizing co-eternal and pre-existing matter but he did this through the un-embodied Jesus and through Adam, a man (a God?) with a body, who had come from another world (this teaching about Adam was taught by Brigham Young but in the early 1900’s it was rejected by the Church and is no longer taught except it has strangely remained a part of the temple Endowment.)
Jesus was born into our physical existence just like any other person is born, except he had God the Father for his father. (This used to be believed quite literally and I was taught this in 1975 as a missionary in the LTM, i.e., God the Father, a physical being, and Mary had a child in the normal physical way…) Jesus thus became the Only Begotten Son of the Father in the flesh. Jesus is NOT the Only Begotten Son from before all time, but in time, and in the flesh. Jesus is NOT God from God, Light from Light, or True God from True God. Jesus is part of creation, the first fruits of that creation. In this respect, Mormonism tends to be a little Arian, a heresy that took control in major portions of the Church in the 4th century.
I explain this to a greater extent on the page for the question, Is the central Christian doctrine of the Trinity found in the Book of Mormon?
Jesus progressed as we all do, except he alone was without sin, and therefore was able to be offered as a sinless sacrifice for our guilt.
Jesus fulfilled his mission on Earth and received all power, dominion, and glory. Jesus earned the right to be another God just like God the Father, an example for the rest of us to follow.
I’ve detected that the LDS church over the last 10 years is moving to embrace Jesus as a pre-existent God (but inconsistently as a God without a body) from the beginning in accordance with the Gospel and Epistles of St. John. This is quite a bit different from Jesus being our “Elder Brother” who becomes a God in time. Actually it is somewhat ironic that the current move of the LDS church back to a belief in Jesus as a real God from the beginning is what the Book of Mormon has always taught, ‘that God himself came down among the children of men, and redeemed his people.’ (Mosiah 15:1, with a change to the tense to put it in the past rather than the future). “And they are One God, yea the very Eternal Father of heaven and of earth”. (Mosiah 15:4) ONE God, not three Gods, not countless numbers of gods, but ONE God! 2 Nephi 31:21 states this about the Godhead: "And now, behold, my beloved brethren, this is the way; and there is none other way nor name given under heaven whereby man can be saved in the kingdom of God. And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end. Amen."
Mormon doctrine is in a constant state of flux – and many Mormons see this as a positive sign that God stills needs a living
prophet on the Earth today.
So much in Mormonism can only be understood in the light of the Roman Catholicism from which it and all other Western versions of
Christianity came. For example, the Blessing of Children as infants, Confirmation, works for the dead, purgatory
(known to the LDS as suffering as Jesus suffered if we don’t repent), the temple, it’s altars, the veil, the hammer
and the thrice knocking to enter God’s presence, the temple covenants of Chastity, Obedience, and Consecration
(this last covenant is known to Catholics as a vow of Poverty). These all came from Catholicism. One hundred years ago,
the LDS even used to have a Catechism patterned after the Catholic Baltimore Catechism. The structure of the LDS church is
that of Catholic Church except, as in the original church, the Catholic Church doesn’t exert centralized control like the
LDS church does. (Who in the pre-existence was in favor of central control and conformity? He who has ears to hear let him hear!)