because we wanted to come to you—I, Paul, again and again—but Satan hindered us.
(1 Thessalonians 2:18 -- ESV)
This is a continuation of the previous verse. Paul recognizes that there is a power that is working against his return to the Thessalonians. That power is the person of Satan. I imagine that Satan is desperately fighting to stop the spread of the gospel message.
What Satan had purposed for evil, God has purposed for good. Satan's hindering of Paul's return to the Thessalonians has resulted in the first letter to the Thessalonians, which has lasted through the years and has benefited the Church, not just in Thessalonica, but across the globe.