Elder Boyd K. Packer, a member of the Quorum of the Twelve Apostles (deceased), wrote:
“If there is a typical response to our missionaries, it is: “I already have a church. One is just as good as another and it doesn’t matter really which one we belong to, or whether we belong to any. We all end up in the same place anyway.”
Surely no one who really thinks would hold to that position. Nevertheless many people accept it when they would not for a moment apply it or relate it to any other phase of their lives. They would not, for instance, take the same position with regard to education. Who would not smile at the statement that all schools are alike, that one is just as good as another, and that a person deserves the same diploma no matter which school he attends, or which course he takes, or for how long!
Would you agree to send students to just any school, taking any variety of courses, and then award them specialized degrees, anything they wanted—in architecture, law, medicine? Such an attitude would suggest that a man (or woman) would be just as good a surgeon by not studying for it as he would by following the prerequisite courses. No person who has given it real substantial thought would take such a position, and no one of us would want to be under the knife of a surgeon who had been trained, or maybe I should say untrained, in such a pattern.
Isn’t it strange, then, that so many are able to apply such a view to religion? They advocate: Go to any school, take any course, or go to no school at all and we’ll all end up in the same place with the same heavenly diploma.
That just isn’t reasonable, nor is it true.
The position that The Church of Jesus Christ of Latter-day Saints is the only true church upon the face of the earth is fundamental. Perhaps it would be more convenient and palatable and popular if we were to avoid this position; nevertheless we are under a sacred obligation and sacred trust to hold to it. It is not merely an admission; it is a positive declaration. It is so fundamental that we cannot yield on this point. . . .
Now this is not to say that other churches, all of them, are totally without truth. They have some truth—some of them very much of it. They have a form of godliness. Often the clergy and their adherents are dedicated and many of them practice well the virtues of Christianity. They are, nonetheless, incomplete. By the Lord’s declaration “they teach for doctrines the commandments of men, having a form of godliness, but they deny the power thereof” (Joseph Smith—History 1:19)
The gospel might be likened to the keyboard of a piano—a full keyboard with a selection of keys on which one who is trained can play a variety without limits: a ballad to express love, a march to rally, a sweet melody to soothe, and a hymn to inspire; and endless variety to suit every mood and satisfy every need.
How short sighted it is, then, to choose a single key and endlessly tap out the monotony of a single note, one or even two or three notes, when the full keyboard of limitless melody and harmony can be played!
How disappointing it is that when the fullness of the gospel, the whole keyboard, is here upon the earth, many churches tap on a single key! The note they stress may be essential to a complete harmony or religious experience, but nonetheless it is not all there is. It isn’t a fulness.
For instance, one taps on the key of faith healing to the neglect of many principles that would bring greater strength than faith healing itself. Another taps on an obscure key relating to observance of …. a key that would sound different indeed if played in harmony with the other essential notes. A key used like that can get completely out of tune. Another repeats endlessly the key that relates to the mode . . . . and now and then taps one or two other keys as though there were not a full keyboard. And again, the very key used, essential as it is, doesn’t sound complete when played alone to the neglect of others. . . .
We do not say that other churches are wrong so much as we say they are incomplete. The fullness of the gospel has been restored. The power and authority to act for God is present with us. The power and authority or the priesthood rests upon this church. . . .
Now the one key danger is not limited to investigators. Some members of the Church who should know better pick out a hobby key or two and tap them incessantly, to the irritation of those around them. By doing this they can dull their own spiritual sensitivities. They can thus lose track of the inspired knowledge that there is a fulness of the gospel and become, individually, as many churches have become: they may reject the fullness in preference to a favorite note. As this preference becomes exaggerated and distorted, they are led away into apostasy.
From ‘Memorable Stories and Parables by Boyd K. Packer’, Bookcraft 1997, p.34-37

