Colossians 21 For I want you to know how great a struggle I have for you and for those at Laodicea and for all who have not seen me face to face,2 that their hearts may be encouraged, being knit together in love, to reach all the riches of full assurance of understanding and the knowledge of God's mystery, which is Christ,3 in whom are hidden all the treasures of wisdom and knowledge.4 I say this in order that no one may delude you with plausible arguments.5 For though I am absent in body, yet I am with you in spirit, rejoicing to see your good order and the firmness of your faith in Christ.6 Therefore, as you received Christ Jesus the Lord, so walk in him,7 rooted and built up in him and established in the faith, just as you were taught, abounding in thanksgiving.
Paul "struggles" in prayer for the Colossians, and the focus of his prayer that they would continue to grow in understanding ad knowledge of Jesus. There's no bigger focus for Paul than seeing more of Jesus, because all the "treasures of wisdom and knowledge" are found in Him. Paul says this so that "no one may delude [them] with plausible arguments" (v4). It's not impossible, but it is unlikely that we would deluded by arguments that are not plausible or that sound ridiculous. The danger is from things that sound right, ideas that seem appealing, things that take the truth and twist it very slightly.
The "plausible" arguments that we face probably come down to Jesus plus something else. Jesus plus material prosperity is what will make you happy. Jesus plus certain moral behaviour will save you. Jesus plus karma is a sensible way to look at the world. Or maybe it's even Jesus minus something. Jesus as a good teacher minus the bits where He says He is God. Jesus minus the teachings we find challenging or don't like.
Paul is reminding the people that it is not Jesus plus anything. It's just Jesus. They recieved the news about who Jesus is and what He has done and that's not something that they move on from, it is how they continue to walk in Him, it's what they put their roots down into and how they become established in the faith (v6-7). Not Jesus plus. Just Jesus.