Before a child can walk, he must learn to stand. Usually the parents teach the child to stand and then to walk. Paul was the spiritual parent of these Thessalonian Christians. He had brought them to faith and now wants them to stand firm. He wants them to grow in maturity despite the spiritual battle that these believers are facing.
Their commitment to prayer and the word as well as submitting to the leadership of Timothy would help them stand firm until the Lord would return. Paul is overjoyed that the Thessalonians are doing just that.
1 Thessalonians 3:8 "For now we really live, since you are standing firm in the Lord."