Notice that Paul sent Timothy, God's coworker in the gospel of Christ
to strengthen and encourage the Thessalonian believers (1 Thessalonians 3:2). Timothy was going back to the city to strengthen people who had already put their faith in Christ, not to seek to win unbelievers to the faith. Yet Paul notes that he was going to minister the gospel.
One common misunderstanding of the gospel is that it is a message focused on conversion. This idea is reflected in the way some churches describe their services. For example, a church’s evening service may be designated as a gospel service,
meaning that it aims to present an appeal to unbelievers, while the morning service may be designated as a teaching service,
with the aim of building up believers. There is, of course, nothing wrong with either of these aims, but to distinguish them by calling one a gospel service is very dangerous.
The gospel is not simply about repenting, believing, and receiving forgiveness as a transaction that gets you in.
The entire Christian message—from the initial call to repent and believe, to the ethical instructions of the New Testament letters, to the promise of our eternal rest—is the message of the gospel. Any message which is not a proclamation of the gospel is not Christian at all.
The gospel is the message about Jesus Christ, God’s Son, as the One who fulfills all the promises, hopes, and expectations of the Old Testament (Romans 1:1–4). The Old Testament sacrificial system could not of itself provide atonement for sin. The sacrifices were really just a reminder of sin and found their fulfillment in the Lamb of God, who truly takes away the sins of the world (Hebrews 10:1–4; John 1:29). The law of God, important as it was as a reflection of God’s will, was beyond Israel’s power to obey; it is only in Christ that God’s people are given a new heart and enabled to do what they could never do without him (Ezekiel 36:22–27; Romans 8:1–4). Without Christ, all the promises and hopes of the Old Testament come to nothing (2 Corinthians 1:20): we cannot know God; we cannot love God; we cannot obey God; we cannot live lives pleasing to God. It is all impossible without Christ.
Because he understood the gospel in this way, Paul sent Timothy to the Thessalonian believers as a minister of the gospel. It was the gospel—the message about Jesus Christ—that would strengthen them as they struggled with persecution. The same thinking lay behind Paul’s exhortation to the Colossians as they wrestled with false teaching: Therefore, as you received Christ Jesus the Lord, so walk in him, rooted and built up in him and established in the faith, just as you were taught, abounding in thanksgiving
(Colossians 2:6–7). And we could go on to consider any aspect of Paul’s teaching throughout his letters: whether he is speaking about marriage (Ephesians 5:22–33), sexual immorality (1 Corinthians 5:1–13), slaves and masters (Ephesians 6:5–9), his watchword is in Christ.
1 Therefore when we could bear it no longer, we were willing to be left behind at Athens alone,