How can a believer live the Christian life? This question has been asked since the Christian church began and it was asked in Colosse. Indeed it is this question that lies behind these verses from Colossians. In these verses Paul responds to what false teachers in Colosse were suggesting on this important issue.
In his first response, Paul deals with the aspect that argued Christians should live like Jews and keep the various rituals mentioned in the Old Testament (vv. 16-17). His rejection of them as valid for Christian living is based on the fact that they are only shadows whereas what Christians need is the substance, Jesus himself.
These practices were pointers to the reality that was going to come; they were like photographs we might have to help us identify a person we are going to meet; so these practices gave spiritual insight concerning the person and work of the coming Messiah. Yet just as once we have met the person, we don’t need the photographs, so having found Jesus the Colossians did not need the shadows.
Old Testament rituals were not part of the New Testament reality any more than a shadow is part of an object. Since the Christians in Colosse had the substance, it was wrong for them to live as if they did not have it.
Of course, we have to be careful that we don’t merely replace Old Testament rituals with another set taken from someone or something else (such as Christian leaders or denominations). I suppose the equivalent that we face is the danger of tradition, of doing something in the present merely because it was connected to something valuable in the past. Paul is making it very clear that Jesus is the substance of Christian living.