The Thessalonians had suffered, but Jesus will return and judge their persecutors, and when he returns, he will also “be glorified in his saints” (1:10). And so, Paul explains in some detail the events of his coming. Through it all they are to remember that they are beloved by the Lord, chosen as firstfruits, saved, sanctified by the Spirit and in truth, and those who will obtain the glory of the Lord Jesus Christ (2:13,14). Knowing these things, i.e. their security in the Lord, they are to stand firm (2:15). Not only are they to stand firm, but they can stand firm because of these things.
Paul asks the Thessalonians to pray for him. He also provides them some counsel about how to live through the times. At the end of the day (to use a significantly overused idiom), they are encouraged by Paul's benediction: “Now may the Lord of peace himself give you peace at all times in every way. The Lord be with you all” (3:16) & “The grace of our Lord Jesus Christ be with you all” (3:18). Good and encouraging words for God's people, at all times, who are looking for the Lord's return.
Yea, amen! let all adore thee,
high on thine eternal throne;
Savior, take the pow'r and glory,
claim the kingdom for thine own.
O come quickly, O come quickly;
alleluia! come, Lord, come. -- Charles Wesley (1758)