The apostle Paul was forced to leave the infant church of Thessalonica before he was confident about the state of their faith. When he “could no longer endure it,” he sent Timothy “to know [their] faith” (1 Thess. 3:1, 5). When Timothy returned with “good news of [their] faith and love” (3:6), Paul was “comforted” (3:7) and said that he could now “live” (3:8). His heart was overwhelmed with such relief, peace and joy that he seemed to have a hard time expressing it. 
“For what thanks can we render to God for you, for all the joy with which we rejoice for your sake before our God…?” (3:9). In other words, “How can we possibly express sufficient thanksgiving in return to God for the joy we have at your faith?”
Have you ever experienced that kind of joy at the faith of another? John wrote, “I have no greater joy than to hear that my children walk in truth” (3 John 4). There is no joy that compares! Let us encourage each other in the faith that we may all rejoice together!