OK, so?
The Catholic Church likewise claims to be the only true church. The various Protestant sects likewise make claims that the other sects (and Catholicism in particular) are in error.
It's one thing to point out error in someone else's beliefs -- like Jesus did when He was talking to the Sadducees (Matthew 22:23). He told them:
29 Jesus replied, “You are IN ERROR because you DO NOT KNOW THE SCRIPTURES OR THE POWER OF GOD<.b>. 30 At the resurrection people will neither marry nor be given in marriage; they will be like the angels in heaven. 31 But about the resurrection of the dead—have you not read what God said to you, 32 ‘I am the God of Abraham, the God of Isaac, and the God of Jacob’? He is not the God of the dead but of the living.” 33 When the crowds heard this, they were astonished at his teaching. (Matt. 22:29-33)
It's another thing to label specific others as the "children of the devil" -- as Jesus did the Pharisees:
John 8:44,47: You belong to your father, the devil, and you want to carry out your father’s desires. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies…The reason you do not hear is that you do not belong to God.
If the Mormons had wanted to, they could have limited their comments to how others were in error.
But actually, once they adopted Joseph Smith's line of 100% apostasy, 100% restoration -- and he was God's personal from-scratch restorer -- they couldn't be satisfied with merely "reforming" the church.
Martin Luther didn't try to start a separate church. Hence, it's called the "reformation" -- not "restoration."
(The Mormons simply have gone all out in trying to "devilize" Christians...see below the asterisks for documentation of this)
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Who are the Christians, and what is Christianity, per Mormonism?
An 'Interview' Across the Generations with the mouthpieces of the Mormon god, the Lds 'prophets' and 'apostles':
Questions re: What is the Devil's role and association with the Christian church per Mormonism?
Q. Who inspires Christians?
A. Joseph Smith: "What is it that inspires professors of Christianity generally with a hope of salvation? It is that smooth, sophisticated influence of the devil, by which he deceives the whole world" (Teachings of the Prophet Joseph Smith, p.270).
* Joseph Smith, Jr.: “…all the priests who adhere to the sectarian religions of the day with all their followers, without one exception, receive their portion with the devil and his angels.” (The Elders Journal, Joseph Smith Jr., editor, vol.1, no.4, p.60).
Q. Where did Christianity -- whatever its form between 100 & 200 AD -->19th century -- originate?
A. Lds "apostle"-turned-"prophet" John Taylor, who was with Joseph Smith when Smith died: Christianity was "hatched in hell" (Journal of Discourses, Vol. 6, page 176)...
* Lds "prophet" Brigham Young: “Brother Taylor has just said that the religions of the day were hatched in hell. The eggs were laid in hell, hatched on its borders, and then kicked on to the earth.” (Journal of Discourses 6:176).
* Taylor: Christianity...is a perfect pack of nonsense...the devil could not invent a better engine to spread his work than the Christianity of the nineteenth century," (Journal of Discourses, Vol. 6, p.167);
Q. Who is classified as part of the "church of the devil" according to the Book of Mormon?
A. BYU professor Kent B. Jackson: "Since whoever does not belong to 'the church of the Lamb of God' belongs to 'the church of the devil,' as Nephi announced then all systems of worship outside of The Church of Jesus Christ of Latter-day Saints would be classified as 'the church of the devil' by Nephi's definition (Kent B. Jackson, "Watch and Remember" etc. from publication By Study and Also by Faith: Essays in Honor of Hugh W. Nibley on the Occasion of His Eightieth Birthday, 3/27/90, vol. 1, p. 87, citing 1 Nephi 14:9-10 from the Book of Mormon)
Q. What is the church of the devil in our day?
A. Lds "apostle" Bruce R. McConkie: "What is the church of the devil in our day, and where is the seat of her power?.... It is ALL of the systems, both Christian and non-Christian, that perverted the pure and perfect gospel....It is communism; it is Islam; it is Buddhism; it is modern Christianity in all its parts. It is Germany under Hitler, Russia under Stalin, and Italy under Mussolini." (The Millennial Messiah, pp. 54-55.)
* Lds "apostle" Bruce R. McConkie: "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." (Doctrinal New Testament Commentary, 3:551)
Q. This last comment of yours? Is that still taught by the official Mormon church in its official curricula in official Institute classes to college students?
Q. So you're saying that...
A.McConkie: "...the great apostate church as the anti-christ...This great antichrist...is the church of the devil." (Apostle Bruce R. McConkie, Mormon Doctrine p.40)
Who are the Christians, and what is Christianity, per Mormonism?
An 'Interview' Across the Generations with the mouthpieces of the Mormon god, the Lds 'prophets' and 'apostles':
Questions Re: How is Christ's church described as it pertains to Mormons and Christians. Questions of legitimacy and authority.
Q. While we all know not every member of a Christian church is a true Christian, what difference is there between Christians in Christian churches and Mormons who reference themselves as 'Christians?'
A. Brigham Young: "Should you ask why we differ from other Christians, as they are called, it is simply because they are not Christians as the New Testament defines Christianity" (Journal of Discourses 10:230).
Q: Do LDS considers themselves one legitimate church among many?
Lds "apostle" Bruce R. McConkie: "Mormonism is Christianity; Christianity is Mormonism; they are one and the same, and they are not to be distinguished from each other in the minutest detail ...Mormons are true Christians; their worship is the pure, unadulterated Christianity authored by Christ and accepted by Peter, James, and John and all the ancient saints." (Mormon Doctrine, p. 513).
Q: Does that mean if all Mormons are Christians then all Christians are Mormons. Is that right?
A: (You weren't listening...I 'just' answered that:) Lds "prophet" Brigham Young: "Should you ask why we differ from other Christians, as they are called, it is simply because they are NOT Christians as the New Testament defines Christianity." (Journal of Discourses 10:230.) Q. Is the Mormon church the only true church then? Are all the other ones false?
A. Lds "prophet" Joseph Smith: "This [the LDS] Church...is the ONLY only true and living church upon the face of the whole earth" (Doctrine and Covenants 1:30).
* Lds "prophet" Ezra Taft Benson, who served in the Eisenhower administration: "This is not just another Church. This is not just one of a family of Christian churches. This is the Church and kingdom of God, the only true Church upon the face of the earth..." (Teachings of LDS prophet Ezra Taft Benson, p.164-165).
* Lds "apostle"-turned-"prophet" John Taylor: "And who is there that acknowledges [God's] hand?… You may wander east, west, north, and south, and you cannot find it in any church or government on the earth, except the Church of Jesus Christ of Latter-day Saints" (Journal of Discourses 6:24).
* Lds "prophet' Spencer W. Kimball: "This is the only true church ...This is not a church. This is the Church of Jesus Christ. There are churches of men all over the land and they have great cathedrals, synagogues, and other houses of worship running into the hundreds of millions of dollars. They are churches of men. They teach the doctrines of men, combined with the philosophies and ethics and other ideas and ideals that men have partly developed and partly found in sacred places and interpreted for themselves" (Teachings of Spencer W. Kimball, p. 421).