Paul has exerted much effort in developing their purity and their understanding of the gospel. Paul continues to encourage the believers in Colossae and Laodicea to understand that Christ is God and that he lives in believers, so they should live in the same way they received him.
Here "face in the flesh" represents the person as a whole. Alternate translation: "all those who have never seen me personally" or "all those whom I have never met face to face"