The Mormon View of Apostasy

Mormon Doctrine, Bruce R. McConkie, (p.44)

With the loss of the gospel, the nations of the earth went into a moral eclipse called the Dark Ages. Apostasy was universal. "Darkness covereth the earth, and gross darkness the minds of the people, and all flesh has become corrupt before my face." (D. & C. 112:23.) And this darkness still prevails except among those who have come to a knowledge of the restored gospel.

Encyclopedia of Mormonism, Vol.1

Joseph Smith in his first vision (1820) was told by Christ that all existing churches had gone astray, both in their teachings and in their practice, although they had "a form of godliness" (JS--H 1:18-19). Thus it was necessary for a "restoration" of the gospel to take place.
In addition, in the Book of Mormon (1 Ne. 11- 14; 2 Ne. 28; cf. Morm. 8), the prophet Nephi. . . .foresaw a "great and abominable church" that persecuted true Christians and the poor. . . .However, this "great church" was not any one specific church, according to Nephi; in his apocalyptic vision there are only two churches, and "whoso belongeth not to the church of the Lamb of God [The Mormon Church—];belongeth to that great church" (1 Ne. 14:10).