See also: D&C 18:20
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) (Bruce R. McConkie, Doctrinal New Testament Commentary, 3:551) — Book of Mormon Student Manual, p. 14
The use of the word church to describe the organization sponsored and directed by Satan has confused some people because they think of the term in the more limited sense of a specific religious organization. But if one thinks of the phrase “the church and kingdom of God,” one has a better concept of what is meant by the church of the devil. It is his kingdom, the sphere of his influence, the whole of his area of power.
Elder Bruce R. McConkie explained that “the titles church of the devil, and great and abominable church are used to identify all churches or organizations of whatever name or nature – whether political, philosophical, educational, economic, social, fraternal, civic, or religious – which are designed to take men on a course that leads away from God and his laws and thus from salvation in the kingdom of God” (Mormon Doctrine, pp. 137-38).
This definition explains why Nephi was taught that “there are save two churches only; the one is the church of the Lamb of God, the other is the church of the devil; wherefore, whoso belongeth not to the church of the Lamb of god belongeth to that great church” (1 Nephi 14:10). In the scriptures Satan’s kingdom is called by various names: Babylon, the great and abominable church, the mother of harlots, the church of the devil, and the kingdom of the devil. (See 1 Nephi 22:22; 2 Nephi 28:18–19; Mormon 8:28–38; D&C 10:56; 18:20)
It is, therefore, the obligation of the Saints to stand against evil wherever it may be found. The method of doing so is clear, according to President Joseph Fielding Smith: “All who go forth to teach should do so in wisdom and not contend with the churches or engage in profitless debates, but teach in the spirit of kindness and try to persuade people to receive the truth” (Church History and Modern Revelation, 1:83). — Doctrine and Covenants Student Manual, p. 35-36
We are ofttimes charged with being very exclusive, and we admit the charge; we are exclusive, but in a rational sense. How can we solemnly testify that this is the Church of Jesus Christ and then ascribe that same high title to other organizations that have been formed not under the direction of Jesus Christ, but according to man’s thoughts and plans? Some people say that we are illiberal because we do not admit that all other churches are what they profess to be, when their profession is based on facts. Now when we say that the Lord is not pleased with those churches, we do not mean that he is not pleased with the members thereof. We hold that God is no respecter of persons, but, on the contrary, that he will acknowledge good in any soul, no matter whether that person belongs to a church or not. But the Lord is not pleased with those churches that have been constructed by men and then labeled with his name. He is not pleased with those doctrines that are being taught as being his doctrines when they are only the effusion of men’s brains, undirected by inspiration and utterly lacking in revelation. — Elder James E. Talmage, Conference Report, October 1928, p. 120