And also those to whom these commandments were given, might have power to lay the foundation of this church, and to bring it forth out of obscurity and out of darkness, the only true and living church upon the face of the whole earth, with which I, the Lord, am well pleased, speaking unto the church collectively and not individually—
However, this statement is structurally ambiguous—it could mean any of a number of different things:
1) The Church of Jesus Christ of Latter-day Saints was the only true church,* and God was pleased with it—the usual Mormon reading, i’d say.
(2) The Church of Jesus Christ of Latter-day Saints was one of more than one true churches, but that God was only pleased with The Church of Jesus Christ of Latter-day Saints.
(3) At the time, The Church of Jesus Christ of Latter-day Saints was the only true church in an active state, and God was pleased with it, but that there were other true churches in some other, perhaps then dormant, state.
(4) God was pleased with many churches, and out of those churches, The Church of Jesus Christ of Latter-day Saints was the only true one.
There are other readings, but these—especially (1) and (2)—seem to me to be the most basic.
Anyway, i think it’s worth pointing out that prooftexts like this can only go so far.
* With “true church” meaning what Mormons generally mean when they say “true church”.