Colossians 2:10 (ESV) … “and you have been filled (complete) in him, who is the head of all rule and authority.”
Paul refers to the believer as being a member of the Lord’s mystical body, the church. Human philosophy and the world’s religions know nothing of that. The word for “complete” is plēroō. It means “to be made full.” We are filled full in Him. Our fulness comes from His fulness. We are mystically so united to Him that we, as members of His body, share His life to the full. This truth is developed more fully in Paul’s companion letter to the Ephesians. In Colossians, the emphasis is on the Lord Jesus as the Head of the body of which we are the members; in Ephesians, the emphasis is on the church as the body of which Christ is the Head.
The truth of all of this we can grasp only falteringly. It is beyond all finite thought. Gnostic notions and supposed secrets are trash and tinsel in contrast to the vast gold mine of truth that we have here. In a coming eternity, all of that fulness and completeness that is ours in God’s Beloved will be displayed, “worlds without end,” to all of the awe-struck created beings in the universe and to the utmost bounds of the everlasting hills throughout all of the ages yet to be.
The cult at Colosse was offering to “complete” the Christian believers by initiating them into their secrets. “Nonsense!” says Paul. We are already complete in Him. At most, what the Gnostics had to offer was some worthless trivia. At worst, it offered access to the deep things of Satan and to forbidden secrets that are dark, dreadful, and damning in nature, effect, and consequence.
 Phillips, J. (2009). Exploring Colossians & Philemon: An Expository Commentary (Col 2:10a). Kregel Publications; WORDsearch Corp.