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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 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.

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.