During the second and third centuries A.D., a number of destructive heresies crept into the churches that threatened to corrupt the biblical canon. False gospels and letters appeared that claimed to have been written by the first-century apostles. Remember the tests that a book had to pass to be accepted into the biblical canon? These books did not pass the tests because they invented stories, disregarded history, and contradicted earlier canonical writings.
The Gnostics of the second century onward believed that all physical matter is inherently evil; therefore, Jesus did not have a physical body. This heresy must have had an early beginning, because the apostle John condemns an early form of Gnosticism as false teaching. The apostle says that he and others saw a physical Jesus, and touched him, after the resurrection (John 1:14; 1 John 1:1).
Docetism denied that Jesus had a physical body, but believed that he seemed to appear in one as a human. The heretical beliefs of Adoptionism stated that Jesus was a mere man whom God adopted and promoted to sonship. Apollinarianism taught that Jesus had a physical body but no human soul or mind. This heretical group said that Jesus’ human flesh was made up of some kind of divine matter instead of bone, flesh, and blood (Luke 24:38–39).
Gatherings of scholars, called councils, met to debate theological issues. In every one, Gnosticism, Docetism, Adoptionism, and Apollinarianism were clearly demonstrated as false and condemned as heresy. Proponents of these teachings were allowed to debate the issue. But in every instance, they contradicted the existing books of the biblical canon. It was very clear that these teachings, and their teachers, were false and dangerous. The false teachings of these and other heretical groups compelled the church to once again confirm the biblical canon.