The chapter begins with Paul reassuring the believers who have not met him personally that what they are believing and following is the truth. He tells them the very reason he is writing: that they would be encouraged and take heart, be united in love, have the full riches of complete understanding & know the mystery of God. This is the key to the epistle. This is Paul's thesis. He wants to make the Gospel clear, and in the clarity of the Gospel we can be encouraged to continue believing.
Christ is central to the Christian faith. Everything begins with Him. Everything ends with Him. Everything consists by Him. We receive Him & continue to live in Him. We are rooted & built up in Him. Therefore, any system of belief that does not center around the person of Christ cannot be the truth. This means if Christ is left out, it is wrong. If Christ is still there, but on the periphery, it is still wrong. Nothing can be true, regarding a faith tradition, without the centrality of Christ.
Paul continues to show the beauty of Christ in all things. He is the fullness of the Godhead bodily. Christ is fully God. He always was and always will be. He never left it anywhere, though he suppressed it for a time. But Christ is not only God, but man. While there never has been a time when Christ was not God, there was a time when He was not man. Christ became man to redeem mankind. Scripture never says that Christ put off his humanity, hence, He is still fully God and fully man. The fullness of the Godhead bodily He is.
Only man could redeem mankind, because man had sinned to bring the curse to mankind. That is why the person of Christ (His nature) is so important. There is a righteous requirement for the redeemer, and Christ satisfies every single one of them.
Next, Paul speaks to the legalistic tendencies that mankind tends to take part in. He begins with the "do's" and ends with the "don'ts."
Paul says not to be held hostage to any ritual. Specifically, do not let people's judgement affect your decisions regarding religious activity. Note that He does not ban any of these things altogether. He simply says not to hang your beliefs on them--because that would push Christ aside. If the ritual draws you to the person of Christ, then it is to be encouraged. Paul says not to focus on "doing" more than focusing on Christ.
Further, Paul continues in the next portion, since we are associated with Christ, we are free enjoy our life on earth. There are things which we would be restricted from in our own human wisdom that there is no need to restrict any longer. Asceticism is not the lifestyle that Christ requires--though there is nothing wrong with performing acts of Asceticism (fasting, denying yourself of a pleasure for Christ). While it is not forbidden, it is not something that should be central, only Christ is central. Paul says not to focus on "avoiding" more than focusing on Christ.