What is a temple?
Mormon temples are houses of the Lord. They are today, as they were anciently, built out of the best materials available. The purpose of temples is to perform ordinances which are necessary to individuals’ salvation. Each ordinance performed in a temple is performed only once for an individual. When a person returns to the temple, he or she is priviliged to perform those ordinances for deceased people who never had the opportunity to hear the gospel in this life. All Church members are encouraged to do their own family history for this purpose, so they can do the work for their immediate ancestors.
Mormon doctrine teaches that families are eternal units which can (and should) be together forever. When ordinances are performed by proxy for those who have passed on, Mormons believe that those individuals still have the choice of whether or not they want to accept those ordinances. Thus, they are not forcing anyone to become Mormon. Rather, they are simply giving them the chance to accept ordinance work which cannot be done in the afterlife; it can only be done on this earth.
A place of instruction
The temple is a place of worship and learning. The endowment ceremony contains an ordinance which teaches individuals more about their relationship to God and how they can live in this life to return to Him in the next: “The temple is a place of instruction where profound truths pertaining to the Kingdom of God are unfolded. It is a place of peace where minds can be centered upon things of the spirit and the worries of the world can be laid aside. In the temple we take covenants to obey the laws of God, and promises are made to us, conditioned always on our faithfulness, which extend into eternity” (The Priesthood and You, Melchizedek Priesthood Lessons—1966, Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 1966, p. 293).
Teachings from the early church
Work for the dead was referenced by Paul: “Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?” (1 Corinthians 15:29). The Mormon Church believes that many such plain and simple truths were lost over time: some in translation, some in the copying of texts, and some by the evil designs of men. Authority and priesthood power were also lost after the apostles were all killed. A restoration was necessary to bring all of these things back, and this restoration happened through Joseph Smith. The priesthood was given to Joseph and to those others the Lord called to serve with him.
“And verily, verily, I say unto you, that whatsoever you seal on earth shall be sealed in heaven; and whatsoever you bind on earth, in my name and by my word, saith the Lord, it shall be eternally bound in the heavens; and whosesoever sins you remit on earth shall be remitted eternally in the heavens; and whosesoever sins you retain on earth shall be retained in heaven” (Doctrine and Covenants 132:46). This sealing power is the same power that Elijah had which will be responsible for turning the hearts of the fathers to the children. This is done through temple sealings. Momron doctrine teaches that we are one eternal family, and that we cannot be saved without our kindred dead. “Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord: And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers, lest I come and smite the earth with a curse” (Malachi 4:5–6).