Very broadly, I was always taught a cult is a group that denies the Trinity, changes the 66-book Bible in any way (including adding to or deleting from), and believes in a salvation other than by grace through faith in Jesus Christ. So, included in this definition would be Mormons, Jehovah's Witnesses, Roman Catholics, Episcopalians (strict Anglicans), and Church of Christ. But I agree that there is no firm definition that is universally agreed upon.

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Mason
Member, Redeemer Presbyterian Church (PCA)
New York, NY

"Come now, and let us reason together," says the Lord, "Though your sins are like scarlet, they shall be as white as snow; though they are red like crimson, they shall be as wool." - Isaiah 1:18