1 Nephi 14:8-11

After the angel had spoken the words about the captivity of the devil and the goals of those that belong to the great and abominable church, he asked Nephi if he remembered the covenants that the Lord had made with his fathers, or Abraham, Isaac, and Jacob. Nephi said yes. The angel shows Nephi again the great and abominable church. He says that the great and abominable church is the mother of abominations and that the founder is the devil.

Next, the angel says that there are save two churches only, one being the church of the Lamb of God and the other the church of the devil. And whoso does not belong to the church of God belongeth to the great church of the devil, the mother of abominations, and the whore of all the earth. I think that verse 11 describes what this means. It says that she sat upon the many waters, and she had dominion over all the earth, among all nations, kindreds, tongues, and people. The cross reference to Jeremiah 51:13 helps to know what it means that 'she sat upon the many waters.' It says:

"13 O thou that dwellest upon many waters, abundant in treasures, thine end is come, and the measure of thy covetousness."

The waters contain many treasures and represents covetousness. These things are right in line with the goals of the great and abominable church.

Here is quote from the Book of Mormon Institute manual about the only two churches:

Elder Bruce R. McConkie defined the church of the devil in the following way: “The church of the devil is the world; it is all the carnality and evil to which fallen man is heir; it is every unholy and wicked practice; it is every false religion, every supposed system of salvation which does not actually save and exalt man in the highest heaven of the celestial world.
It is every church except the true church, whether parading under a Christian or a pagan banner. As Moroni will say in a later era of Nephite history, and as we shall ascertain in our evaluation of Rev. 18:1–24, it is ‘secret combinations,’ oath-bound societies, and the great world force of Godless communism. (Ether 8:14–26.)” (Doctrinal New Testament Commentary, 3:551).