From the opening words of the epistle, we learn that the church in Colosse had its inception in sound doctrine.
This is a necessity for any Christian church. The church at large is built on good doctrine. When Peter confessed that Jesus was the Christ, the Son of the living God, Jesus said He would build His (yet-future) church on that foundation: on the confessed truth of His deity (Matthew 16:13-18). As He predicted, so it happened. The inaugural sermon of the Christian church was the preaching of Christ, leading to a mass confession of Christ as Lord (Acts 2).
As it is with the universal church, so it is with any local church. A church of Christ must be build on the foundation of the preached truth of Jesus Christ (1 Corinthians 3:10-11). Paul says categorically that no other foundation can be laid than Jesus Christ.
What does it mean to lay a foundation of Jesus Christ? Clearly Paul does not mean that he is pulling Jesus down from the Father's right hand, and constructing an edifice on Him. Obviously what Paul must mean in-context is Jesus Christ as preached and believed. It is the true doctrine of Jesus Christ that is laid as a foundation by preaching; it is the confession of Christ as preached by the apostles that is the foundation of the church, of any Christian church. Without that, there can be no church of Christ.
The church of Christ, then, is founded on doctrine, on theology, specifically on Christology.
This was the start that the Colossians got. See 1:4-7, where Paul speaks of
having heard of your faith in Christ Jesus, and the love which you have for all the holy ones, 5on account of the hope which is laid away for you in the heavens, of which you heard before in the word of the truth, the good news, 6which has come to you, just as also in all the world it is bearing fruit of itself and growing just as also among you, from the day in which you heard it and came to know fully the grace of God in truth; 7just as you learned it from Epaphras, our beloved fellow-slave, who is a faithful servant of Christ for your sake... (DPUV)So far, so good.
However, the little church had run into a problem. As it had begun by sound doctrine, so now it was being threatened by sick doctrine.
Who or what was behind it? It is difficult to say, impossible to be absolutely certain. Paul doesn't name some aberrant cult or sect. He doesn't talk about Jehovah's Witnesses, Mormons, Roman Catholics, Christian Scientists, Gnostics, or any other identifiable false sect. We are left to piece things together from the indications within the letter, both the subtle and the obvious.
Commentators tend to assume that a group of false teachers were exerting a baleful influence. It is common to read of “the false teachers” in Colosse, and of "the Colossean heresy." Even more, it is very common for writers and preachers to identify them positively as Gnostics, a movement visible in the second century and beyond.
Here's where I strike out on my own, with a position I haven't seen anyone else take. This probably means I'm wrong. But I'll tell you what I think, and why I think it, leaving you to make your own judgment.
Next time, Lord willing.