http://bible.org/article/non-pauline-epistles
From the book of Jude:
The Gnostics viewed everything material as evil and everything spiritual as good. They therefore cultivated their “spiritual” lives and allowed their flesh to do anything it liked, with the result that they were guilty of all kinds of lawlessness.
The “false teachers” idea was apparently a big deal, because we now have multiple epistles written to contend with it. The early Christian church was probably beset with many influences from the cultures around it. Some thought they could take the tenets of Christianity as the latest hot thing, and wrap their ideas around it. This would corrupt and warp the original teaching, which is why the apostles were so quick to point out the deceitful nature of the false teachers.