This section of Colossians is tightly packed with rich theological truth!  Paul unfolds for the church the crucial significance of the mystery of Christ in his ministry and his personal experience of suffering. Look at v. 24: "Now I rejoice in my sufferings..."  Joy in suffering, not just joy on the other side of suffering.  Remember that Paul penned this Christ-centered letter while he was in prison (cf. Col 4:3). What is Paul's secret for joy and ministry in the midst of such adversity? Simply put, Christ is the secret.  Paul's Christology completely transforms his understanding of his suffering, as he realizes that God's design for his suffering is to "fill up what is lacking in Christ's afflictions for the sake of his body, that is, the church." What is lacking in Jesus' afflictions is not propitiation but presentation. As he suffers, he is laboring "to make the word of God fully known" (v. 25). Paul's own sufferings are designed to show off his suffering savior to a world that so desparately needs Jesus; he is in his own experience of suffering offering a personal presentation of the Jesus he proclaims (v. 28).

The main point of this passage is in v. 28a: "Him we proclaim!"  Paul's proclamation of Christ is carried out in a 2-fold manner: by warning and teaching everyone, with the goal that he may present everyone mature in Christ.  Paul in this very paragraph warns the church against deceivers with enticing words (πιθανολογίᾳ) (2:4), while he instructs the church with all wisdom about the person of Christ and the nature of his Christ-centered ministry of suffering for the sake of the church.  Paul's goal is reiterated 2:2-3 ("to reach all the riches of full assurance of understanding and the knowledge of God's mystery, which is Christ...").

Doug Moo suggests that this passage is structured in a chiastic pattern (= A, B, B, A)  (p. 148):

A Rejoice ... flesh   1:24

   B make known ... riches ... mystery   1:27

      C  contend   1:29

      C'  contending   1:21

    B' knowledge ... riches ... mystery   2:2

A'  delight ... body   2:5

This shows Paul's carefully crafted argument for the supremacy of Christ which employs various repeated words (such as 'mystery' [μυστήριον] and 'rejoice' [χαίρω]), but the disadvantage of this presentation is that it does not put as much stress on the logical connections in the text, such as 'for' [γὰρ] in 2:1. I see vv. 1-5 offering both personal and theological grounds for Paul's claim in vv. 28 that he proclaims Christ for the purpose of presenting believers mature in Christ.