intruding into those things
The errorists against whom Paul warns the Colossians, and against whom, in principle, the warning has perpetual significance, were called "Gnostics," from the Greek, "gnosis", meaning "knowledge."
These Gnostics "came most keenly into conflict with the exalted rank and redeeming rank of Christ, to whom they did not leave His full divine dignity, but assigned to Him merely the highest rank in the order of spirits, while they exalted angels as concerned in bringing the Messianic salvation." -- H.A.W. Meyer.
Paul's characteristic word, in Colossians, for the divine revelation is the Greek, "epignosis", that is, "full knowledge", (Colossians 1:9); (Colossians 1:10); (Colossians 3:10) as against the pretended "knowledge" of the errorists. The warnings apply to all extra-biblical forms, doctrines, and customs, and to all ascetic practices.
angels
( See Scofield) - (Hebrews 1:4).