Specifically, we are talking about the sufficiency of the cross to give a person access into the presence of God. It has to do with the rending of the veil from top to bottom. Paul’s letter to the Colossians, the entire letter, is about the sufficiency of our Savior and His salvation, His person and work.
Colossians was written to churches that were contending for the gospel, that were being told that the gospel was insufficient in bringing people into such a relationship with God. They needed something more. Whatever the “more” was, it involved philosophy and empty conceit, according to the traditions of men, according to the basic principles of the world, and not according to Christ (Col. 2:7). The specifics of what “more” was needed included standards involving food or in drink, or regarding a festival or a new moon or sabbaths (2:16). Since Paul says these things are a shadow of things to come, but the substance is of Christ (2:17) it is possible there was some Jewish/Old Testament influence involved, although it does not appear to be the strong “Judaizing” influence experienced by the Galatian Churches.
In addition, from 2:18-23, the “more” involved taking delight in false humility and worship of angels … regulations – “do not touch, do not taste, do not handle.” These were according to the commandments and doctrines of men, and had an appearance of wisdom in self-imposed religion, false humility, and neglect of the body. Since Paul does not speak of these in detail, nor does he contradict each regulation, it is possible that the problem is not that they have been led to follow the wrong “religious practices” but that they have been tempted to follow ANY religious practices.
It is also clear that what the Colossian believers are being called to “do” is seen by Paul as “in addition to” the gospel. In Col. 2:8 these things are “not according to Christ.” In 2:17 the religious practices are at best “a shadow” but the substance is of Christ. Doing these things has kept them from not holding to the Head, from whom all the body, nourished and knit together by joints and ligaments, grows with the increase that is from God (2:19). In 2:20 he clearly states that doing these things is totally unnecessary if you died with Christ from the basic principles of the world. And at the end (2:23) he is emphatic that these religious practices are of no value against the indulgence of the flesh. In other words, these religious practices, being designed to teach discipline or self-control, which is evident in all the rules and restrictions and taboos, are useless to do the very thing they are intended to do.
What is described here is not unusual. It sounds like, essentially, every religion ever created. Paul rightfully relates this to “traditions of men” and “basic principles of the world.” The New Testament never taught these things. We were not left with a system of religion, not even in the physical observances of water baptism and the Lord’s Table (nor foot washing if that is one of your practices). There is a reason for that, and we will see it in Colossians.