The Latter-day Saints believe that in order to attain the best in life and the greatest happiness in this world and for the next, men and women must be married in the temple for time and eternity. Without the sealing ordinances of temple marriage, man cannot achieve a godlike stature or receive a fulness of joy. …
To a Latter-day Saint, there is only one kind of marriage that is wholly acceptable: temple or celestial marriage, which is performed only in the temples of the Church. Temples are erected and dedicated in holiness to the Lord to provide a place where spiritual and eternal ceremonies and ordinances may be performed. While we recognize civil marriages performed by ministers of other churches and civil marriages performed by officers of the law or others legally qualified to perform them, we believe that only in a temple of God can a marriage for time and eternity be performed and then only by one having the authority which Christ gave to Peter when He said, "Whatsoever thou shalt bind on earth shall be bound in heaven" (Matthew 16:19).
This authority is referred to in the scriptures as "the keys of the kingdom of heaven" (Matthew 16:19), and in celestial marriage those keys open the door to that kingdom.