Basically, their teaching seems to have been Jewish. This appears from the part played in it by legal ordinances, circumcision, food regulations, the sabbath, new moon and other prescriptions of the Jewish calendar. But it was not the more straightforward Judaism against which the churches of Galatia had to be put on their guard. That Judaism was probably introduced into the Galatian churches by emissaries from Judaea; the Colossian heresy was more probably a Phrygian development in which a local variety of Judaism had been fused with a philosophy of non-Jewish origin—an early and simple form of gnosticism.1
…All this was presented as a form of advanced teaching for a spiritual élite. The Christians of Colossae were urged to go in for this progressive wisdom and knowledge (gnōsis), to explore the deeper mysteries by a series of successive initiations until they attained perfection (teleiōsis). Christian baptism was but a preliminary initiation; those who wished to proceed farther along the path of truth must put off all material elements by pursuing an ascetic regimen until at last they became citizens of the spiritual world, the realm of light.
F. F. Bruce, Paul: Apostle of the Free Spirit (Milton Keynes, UK: Paternoster, 1977), 413, 414.
NOTE
1. HB editor’s note: It seems rather more accurate to say that the Colossian heresy contained within it some of the same impulses and errors found in Gnosticism, which was a second-century phenomenon.