It's not just me saying it:
Certain Christian doctrines constitute the core of the faith.
"Central doctrines include the Trinity, the deity of Christ, the bodily resurrection, the atoning work of Christ on the cross, and salvation by grace through faith. These doctrines so comprise the essence of the Christian faith that to remove any of them is to make the belief system non-Christian."
Alan W. Gomes. Unmasking the Cults.
"Therefore for orthodox Christianity, cults of Christianity are groups that, while claiming to be Christian, deny central doctrinal tenets such as the Trinity and the deity of Jesus Christ. They deviate from the doctrinal norms set forth in the »Bible and the historical creeds of Christendom."
Nichols, L. A., Mather, G. A., & Schmidt, A. J. (2006). In Encyclopedic Dictionary of Cults, Sects, and World Religions (p. 381). Grand Rapids, MI: Zondervan.