Paul and his friends gave their souls and lives to the conversion of the Thessalonians, but then persecution arose against the preachers and they had to leave.
The converted Thessalonians also received persecution, and so Paul wondered whether they remained faithful. When he learned that they had been, he wrote to them,
“For this reason, when I could no longer endure it, I sent to know your faith, lest by some means the tempter had tempted you, and our labor might be in vain” (1 Thessalonians 3:5).
Let us convert people, but then be sure to continue to look out for their welfare that they not fall back into that from which they came.