The Sealing Power

The temples have become a powerful symbol representing the everlasting covenant. Because the Spirit of the Lord is in the temple, the temple becomes a place where we can appear before the Lord. In the Old Testament, Israel was instructed to appear before the Lord three time a year. They all knew that this meant for them to go to the temple on those occasions.

In latter-day scripture the Lord explains that the everlasting covenant is to be the rallying point for the righteous,

D&C 45:9-10
9 And even so I have sent mine everlasting covenant into the world, to be a light to the world, and to be a standard for my people, and for the Gentiles to seek to it, and to be a messenger before my face to prepare the way before me.
10 Wherefore, come ye unto it, and with him that cometh I will reason as with men in days of old, and I will show unto you my strong reasoning.

Holy sanctuaries wherein sacred ordinances, rites, and ceremonies are performed which pertain to salvation and exaltation in the kingdom of God are called temples. They are the most sacred places of worship on earth; each one is literally a House of the Lord, a house of the great Creator, a house where he and his Spirit may dwell, to which he may come, or send his messengers, to confer priesthood and keys and to give revelation to his people.
(Bruce R. McConkie, Mormon Doctrine, 2d ed., p.779)

The sealing power exercised in the temples is the most sacred of all the powers given to man,

On this same occasion (when the Prophet gave them their final charge), according to parley P. Pratt, Joseph conferred "the keys of the sealing power" on Brigham Young, President of the Twelve. The Prophet taught them that it was the "last key," the "most sacred of all," and that it pertained "exclusively to the first presidency." (Joseph, Brigham and the Twelve: a Succession of Continuity by Ronald K. Esplin Fn, BYU Studies, vol. 21 (1981), Number 3 - Summer 1981, p.244)

President Marion G. Romney explained the importance of temples and families being sealed together,

Temples are great fortresses for righteousness in the world. The devil opposes them. He so stirred the enemies of our people after the first temples were built that the Saints were required to move away from Kirtland and Nauvoo. Without the great outpouring of Spirit and power given in those temples, it is doubtful whether the Church could have survived. President Stephen L Richards, at the dedication of the Los Angeles Temple, spoke about the power of the sealing ordinance reaching beyond our understanding. It is my belief that the power emanating from temples is far greater than we realize.
In addition to being a place for the Lord to "reveal unto his people the ordinances of his house and the glories of his kingdom, and teach the people the way of salvation" (History of the Church 5:423), a temple is a place where the living Saints receive the higher ordinances of the priesthood necessary to their exaltation.
We all know what the necessary ordinances of the gospel are: first, baptism; second, the laying on of hands for the gift of the Holy Ghost. These ordinances are administered to the living in places other than a temple, as is also the third ordinance, the laying on of hands for the bestowal of the priesthood. The higher ordinances of the gospel, however, those pertaining to the endowment and the sealing ordinance, can only be performed in temples.
The Prophet Joseph made it clear that men could not be saved-meaning exalted-unless they had these ordinances performed for themselves.
I would advise all the Saints to go to with their might and gather together all their living relatives to this place, that they may be sealed and saved, that they may be prepared against the day that the destroying angel goes forth. . . .
The question is frequently asked, "Can we not be saved without going through with all those ordinances, etc.?" I would answer, No, not the fullness of salvation. Jesus said, "There are many mansions in my Father's [kingdom] and I will go and prepare a place for you." . . . And any person who is exalted to the highest mansion has to abide a celestial law, and the whole law too (History of the Church 6:184).
In temples we are sealed for eternity to our wives, our husbands, our children, and our ancestors. Our leaders repeatedly speak of the home as the center of a Latter-day Saint life. Without the sealing of husbands and wives, children and parents, there would be no family relationships in the world to come; there would be no eternal homes. How terrible this would be. Without the home, heaven would be devoid of its happiness. Wherever my beloved wife and children are is heaven to me. I therefore consider the Salt Lake Temple, in which the sweetheart of my youth and I were sealed together for time and eternity, as the gate to heaven for me.
(Marion G. Romney, Look to God and Live , p.236-237)