Some translations (e.g., the NIV) understand these words as the spreading of the gospel of Christ, but this is an interpretation of the original words, which are simply in the gospel of Christ.
The NIV translators have translated the phrase in such a way as to imply that the gospel work in which Timothy was involved was the work of evangelism.1 However, in Paul’s mind all the work of ministry—whether aimed at conversion or Christian growth—was gospel work (cf. Romans 1:1–5). Thus, Timothy was God's fellow worker in the gospel of Christ.
By thus linking the idea of being God’s coworker with that of the gospel, Paul takes us to the heart of his understanding of ministry and God’s saving work. It is valuable to expand on this briefly.
For Paul, the gospel was the message about God’s saving work in Christ (Romans 1:2–4). The gospel proclaimed the coming of Christ into the world to inaugurate the new age of salvation foretold in the Old Testament. This age is characterized by the new and powerful working of God in Christ to accomplish all that had been hoped for throughout the Old Testament period (2 Corinthians 5:17). Human beings receive God’s salvation and participate in the new age through faith in Christ as he is set forth in the gospel. While the gospel is proclaimed by people, it is, in fact, a divine message and God is powerfully at work through it. Through the gospel he both calls people to Christ and transforms them into the image of Christ. Hence, Paul declares: I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes
(Romans 1:16, emphasis added).
The character of gospel ministry flows from the nature of the gospel itself. As Paul wrote the words of 1 Thessalonians 3:1–13, he was engaged in preaching the gospel in Corinth, preaching which he later described by saying, I decided to know nothing among you except Jesus Christ and him crucified...so that your faith might not rest in the wisdom of men but in the power of God
(1 Corinthians 2:2–5). The preaching of the gospel is thus an event in which the labour of man and the power of God unite in a remarkable way which is clearly and powerfully described in 2 Corinthians 5:18 – 6:1:
"All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation: that God was reconciling the world to himself in Christ, not counting people's sins against them. And he has committed to us the message of reconciliation. We are therefore Christ’s ambassadors, as though God were making his appeal through us. We implore you on Christ’s behalf: Be reconciled to God. God made him who had no sin to be sin for us, so that in him we might become the righteousness of God. As God’s co-workers we urge you not to receive God’s grace in vain" (NIV, emphasis added).
By describing Timothy as God’s coworker in the gospel of Christ, Paul shows that he expected Timothy to do in Thessalonica what he himself was doing in Corinth: he was to preach the gospel of Christ in such a way that the saving power of God would be active in the lives of the Thessalonian believers. Though many interpreters of Paul have been uncomfortable with the idea of Timothy being God’s coworker, that concept is, in fact, at the heart of gospel ministry as Paul understood it.
2 and we sent Timothy, our brother and God’s coworker in the gospel of Christ, to establish and exhort you in your faith,