Answer
The Colossian heresy was the false teaching that was being propagated in the Colossian church and that caused Paul to write his epistle to the church. Paul never specifically says that there is a heresy that he is combating, but by reading the contents of his letter we can discern that there was a false teaching that some in the church were embracing or at least considering.
Paul begins the letter by emphasizing the superiority of Christ. Of course, this could be the subject of the epistle without being a response to a specific heresy, but the emphasis seems more pronounced than in any of his other letters, so most scholars assume that the Colossian heresy in some way diminished the person of Christ.
Colossians 1:15–20 says of Christ, “He is the image of the invisible God, the firstborn of all creation. For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. And he is before all things, and in him all things hold together. And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent. For in him all the fullness of God was pleased to dwell, and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross” (ESV).
From there, Paul moves to the topic of the gospel, which is based on the person of Christ, and the proper response of faith. In Colossians 2, Paul warns the church against being fooled by human philosophy and tradition because it is in Christ that “all the fullness of deity dwells bodily” (Colossians 2:9, ESV). It is in Christ that sins can be forgiven and a person be made right with God. Therefore, the church should not allow anyone to “pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath. These are a shadow of the things to come, but the substance belongs to Christ. Let no one disqualify you, insisting on asceticism and worship of angels, going on in detail about visions, puffed up without reason by his sensuous mind, and not holding fast to the Head, from whom the whole body, nourished and knit together through its joints and ligaments, grows with a growth that is from God” (Colossians 2:16–19, ESV).
Apparently, some who claimed to embrace Christ were allowing themselves to be controlled by legalistic principles. They may have been under pressure from Jewish sources or other groups promoting asceticism. “If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations—‘Do not handle, Do not taste, Do not touch’ (referring to things that all perish as they are used)—according to human precepts and teachings? These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh” (Colossians 2:20–23, ESV).
The rest of the epistle to the Colossians` gives practical instruction to Christians on good behavior. This behavior is based on their position in Christ and their desire to live a godly life, but it is not the basis of their acceptance before God. The Christian life does have principles for living, but these are based on love for God and neighbor, not on religious rules and rituals.
Drawing primarily on the information in the first part of Colossians, we surmise that the Colossian heresy diminished the preeminence of Christ and the sufficiency of His sacrifice on the cross to forgive sins. Instead, this false teaching emphasized adherence to rules and regulations that are powerless to truly change lives. Most modern cults also diminish Christ and emphasis rituals, so the message of Colossians is timely even now in the 21st century.
