When I was a child on sunny days I liked to play with my shadow. My friends and I would make shadow puppets, play shadow tag, and try to guess what animals the other was making with their hands.
It would be a rather silly thing to do the same thing as adults.
Yet that is what the false teachers in Colossae were suggesting:
So don’t let anyone pass judgment on you in a question of food or drink, or in the matter of festivals, new moons, or sabbaths. These things are a shadow cast by the coming reality — and the body that casts the shadow belongs to the king. (2:16-17)
For Paul, the best kind of knowledge is the relational understanding of knowing Jesus, not just the esoteric knowledge of the false teachers for whom Jesus was more of a concept than a person (2:2-3).
Paul also emphasizes the relational side of baptism, something some of us today may have forgotten too. Baptism is first and foremost about being buried and raised with Jesus (2:12). Water baptism is always imitation of and a mysterious participation in the more important reality of Christ’s death, burial and resurrection; it is never the point itself.
Maybe that is a sign of all false teaching: it promotes a religion without a full and personal relationship with Jesus.