Although love
could refer to love for God, for other people in general, or for believers—or to all three—here are two reasons to think that Paul is referring here to love for fellow believers.
This letter has a strong emphasis on brotherly love (1 Thessalonians 3:12; 1 Thessalonians 4:9–10; 1 Thessalonians 5:13), which is understood to come from God (1 Thessalonians 4:9). The introductory thanksgiving foreshadows the themes of the letter (see above on the nature of the thanksgiving), and therefore it is natural to understand the love of 1 Thessalonians 1:2 as a reference to brotherly love, which is addressed later on in the letter.
In Colossians 1:4–5, another introductory thanksgiving, Paul speaks explicitly of
faith in Christ Jesus and the love you have for all the saints—the faith and love that spring from the hope that is stored up for you in heaven
. Here again we find the mention of faith, love and hope, as in 1 Thessalonians 1:3. In the Colossians passage faith is explicitly related to Christ and love to fellow believers. It reasonable to think that Paul’s mind was moving along similar lines in both thanksgivings.
We may therefore conclude that labor of love
refers to sacrificial works of service done by the Thessalonian Christians for one another and for believers from other places.1
3 remembering before our God and Father your work of faith and labor of love and steadfastness of hope in our Lord Jesus Christ.