Matt Slick is the founder and president of Christian Apologetics and Research Ministry. He wrote the following article: Mormonism in a Nutshell.
Mormonism teaches that God used to be a man on another world, and that he became a god by following the laws and ordinances of his god on his home world. He brought his wife to this world, a woman he had married on the other world. She is essentially a goddess.
In his present god-state, he rules our world. He has a body of flesh and bones. Since god and his wife are both exalted persons, they each possess physical bodies. In their exalted states as deities, they produce spirit children that grow and mature in the spiritual realm. The first spirit born was Jesus. Afterwards, Lucifer was born along with the rest of us. So, Mormonism teaches that we all pre-existed in the spirit realm–having been produced from the union of god and his goddess wife. Therefore, we all existed in spirit form before coming down and entering the bodies of human babies that are being born on earth. During this ‘compression’ into the infant state, the memories of our pre-existence is ‘veiled.’
According to Mormonism, this god’s name is Elohim
God the father, who is called Elohim, was concerned for the future salvation of the people on earth. In the heavenly realm, the Father had a plan for the salvation of the world. Jesus endorsed the Father’s plan. Lucifer did not. Lucifer became jealous and rebelled. In his rebellion, he convinced a large portion of the spirits existing in heaven to side with him and oppose god. God, being more powerful then they, cursed these rebellious spirits to become demons. They can never be born in human bodies.
The remaining spirits sided with God. Since they chose the better way, when it comes time for them to live on earth, they have the privilege of being born in races and locations that are relative to their condition and choice made in the spirit realm.
The Plan of Salvation
In the Mormon plan of salvation there needed to be a savior: Jesus. But Jesus was a spirit in heaven. For him to be born on earth, Brigham Young, the second prophet of the Mormon church, said that instead of letting any other man do it, God the Father did it with Mary. He said that the birth of our savior was as natural as the birth of our parents. Essentially, what this means is that Brigham Young taught that god the father came down and had relations with Mary, his spirit daughter, to produce the body of Jesus. Though many Mormons will not entertain such incestuous thoughts about God and Mary, this is what Brigham Young taught; and as far as we know, this has not been denied by the Mormon church.
Nevertheless, Jesus was born, got married, and had children. He died on the cross and paid for sins–but not on the cross only. According to Mormonism, the atonement of Christ was not only on the cross. It began in the Garden of Gethsemane before he went to the cross.
BYOG (Be Your Own God)
In Mormonism, men and women have the potential of becoming gods. President Lorenzo Snow said, “As god once was, man is. As God is, man may become.” In order to reach this exalted state of godhood, a person must first become a good Mormon, pay a full ten percent tithe to the Mormon church, follow various laws and ordinances of the church, and be found worthy. At this point, they receive a temple recommend, whereupon the Mormon is allowed to enter the sacred temples in order to go through a set of secret rituals: baptism for the dead, celestial marriage, and various oaths of secrecy and commitment. Additionally, four secret handshakes are taught so the believing Mormon, upon entering the third level of Mormon heaven, can shake hands with god in a certain pattern. This celestial ritual is for the purpose of permitting entrance into the highest level of heaven. For those who achieve this highest of heavens, exaltation to godhood awaits them. Then he or she will be permitted to have his or her own planet and be the god of his own world, and the Mormon system will be expanded to other planets.
Please see Mormon Beliefs, are they Christian? for further documentation of Mormon beliefs.