Suffering (24-25a): Paul is a minister/servant[1] who suffers for Christ's church. But he rejoices in this (1:24), because he saw it as a privilege to fill up "what is lacking in Christ's afflictions." Paul does not mean by this that he can add anything to Christ's finished work of redemption.[2] He is referring, instead, to how Christ lets His servants offer their blood and sweat ("in my flesh"), even their sufferings, for the building up of His church.[3] And this opportunity is for others as well.[4] Suffering for the good of God's people is our privilege!
The Word (25b-28): Paul's ministry is "the preaching of the word of God," which is the message about Christ (1:28, "we proclaim Him"). This is the gospel, hidden in the past but now revealed (1:26), a message which calls all the nations to a new life under Christ's lordship (1:27).[5] This proclamation of Christ points people to future glory (1:27, "the hope of glory") and is fully able to transform people unto spiritual maturity (1:28, "complete in Christ").
Empowerment (29): Now, all credit must go to God when we labor and strive for the good of His church. Paul personally testifies of this. His hard work is "according to His power which mightily works within me" (1:29). And this is no different for the everyday saint who gives of himself in service to the Lord (1 Pet. 4:11; 2 Thess. 1:11-12). God supplies the strength and endurance!
[1] Paul calls himself a servant/minister (di-a-ko-nos [διάκονος]) in 1:23 and 1:25. He sees himself as serving.
[2] He's already established that Christ's death effectively redeems and reconciles us to God (1:14, 20, 22).
[3] This is edifying believers and evangelizing the lost to bring the saved into God's church (cf. Eph. 4:11-12).
[4] Paul calls Timothy to join in suffering for God's church (2 Tim. 1:8, 12; 2:3; 3:10-11). We are even called to lay down our lives for the brethren (1 John 3:16; cf. also Rom. 16:3-4 and Phil. 2:30).
[5] "Christ in you" is the new life under Christ's lordship (Gal. 2:20; Rom. 8:10; 2 Cor. 13:5). Cf. also 1:13.