five minute brief
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LDS doctrine uses Christian vocabulary while teaching a different God, a different Christological framework, and a different authority structure from the Christianity taught by Scripture and confessed by Christian churches.
Christian has a doctrinal meaning
If Christian means only a person or group that says Jesus is important, then the word loses most of its theological force. The Christian boundary is tighter: one God, the Father, Son, and Holy Spirit as the one divine being, Jesus as eternally God, and the apostolic gospel as the church's rule of faith.
Catholics, Orthodox, and classical Protestants disagree on many questions. They still recognize the Nicene center. LDS doctrine explicitly rejects that center.
Official LDS sources state the difference
Official LDS sources teach that Father, Son, and Holy Ghost are distinct beings, that the Father has a body of flesh and bones, that humans can receive exaltation, and that Christian churches lost priesthood authority.
Those claims are enough. They change the doctrine of God, Christ, salvation, scripture, and church authority.
Scripture gives the test
Isaiah says no God was formed before or after the Lord. John says all created things were made through the Word. Galatians says even an angelic message is rejected if it brings a different gospel.
That is the public test. A testimony, prophet, restored book, or institution cannot be allowed to redefine the apostolic faith and then serve as its own proof.
Primary references
The argument rests on public Scripture, official LDS material, and Christian sources.
Godhead, Topics and Questions
The Church of Jesus Christ of Latter-day Saints
Official LDS teaching that the Father, Son, and Holy Ghost are distinct beings, one in purpose and doctrine.
Doctrine and Covenants 130
The Church of Jesus Christ of Latter-day Saints
Canonical LDS text teaching that the Father and Son have bodies of flesh and bones.
Becoming Like God
The Church of Jesus Christ of Latter-day Saints
Official essay explaining LDS divine potential and acknowledging that LDS teaching goes beyond most contemporary Christian churches.
Apostasy, Topics and Questions
The Church of Jesus Christ of Latter-day Saints
Official LDS source teaching that priesthood authority was withdrawn after the apostles.