Innumerable cults have invaded the Church of Jesus Christ and have managed to deceive people – even the Elect, with another “man / woman” or another “book” around which they build their doctrines.
o Roman Catholic – is not Christian, it is Maryian
o Jehovah witnesses deny the deity of Jesus Christ
o Seventh day Adventists follow legalism who want to hold on to the Sabbath to earn God’s favour – not depend on His grace.
o Mormons bring in another “Bible” alongside the Scriptures and turn the grace of God to lasciviousness.
o Unitarians deny the trinity of the Godhead thereby denying the manhood of the Lord Jesus Christ.

There are a lot more - these will be added to this answer in due course.