Understanding that the church is a part of the body of Christ (or, if you prefer, part of the being of Christ), then Christians have a role in living out this unity and fullness of God's creation. To do so, they need to abandon the human ways of this world and adopt the spiritual ways of living embodied by Jesus -- in other words, they need to refrain from committing human sins and instead demonstrate and share spiritual gifts.
If this sounds similar to the message of other New Testament epistles, it is. In fact, almost all of the verses in the third chapter of Colossians can also be found elsewhere. This repetition, though, demonstrates how important it was to the church to understand the difference between a Christian way of life and every other way of life.