when he comes to be glorified in his saints, and to be admired among all those who have believed (because our testimony to you was believed) in that day.
Contextually, it looks as if he does so because he is writing from the perspective of the judgment day. The verse is teaching that Jesus will come to receive glory and wonder and admiration for all He has done for His people, His saints. Paul assures the Thessalonians they, too, will be with Him because they believed God’s word. The same benefits apply to everyone who hears and obeys the word of God. Those who hear and obey are described as wise men who build a house upon a rock (Matthew 7:24-27).