What does 2 Thessalonians 3:4 mean?

Paul's statement about the Thessalonians in this verse must have encouraged them immensely. He expresses confidence in the Lord that they are doing and will continue to do what he commands. This confidence was not based on human merit. It was based "in the Lord." Left to themselves, the Thessalonians would not be able to carry out Paul's commands, but the Lord was working in their lives to help them lead a righteous life. Paul exhorted the Philippians to work out—not "work for"— their salvation with fear and trembling (Philippians 2:12), but he added, "For it is God who works in you, both to will and to work for his good pleasure" (Philippians 2:13).

A relationship with God does not guarantee a life free from disagreement or controversy, however. In his letter to the Romans Paul addressed a situation in the church in which believers had different opinions about what they could eat. Some believed they could eat anything, probably including meat once offered to idols, without having a guilty conscience. Others abstained from eating meat, and were judged as "weak in the faith" (Romans 14:1). Each side apparently criticized the other. But Paul asked , "Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand" (Romans 14:4). We can trust the Lord to perfect His work in our lives and in the lives of our fellow believers.