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.
"That day" is a clear reference to the day of the Lord (cf. Isaiah 2:11; Isaiah 2:17). It will include Jesus Christ’s return to the earth at His second coming (cf. Mark 13:32; Mark 14:25; Luke 21:34; 2 Timothy 1:12; 2 Timothy 1:18; 2 Timothy 4:8). [Note: Thomas, p. 314. Cf. George Milligan, St. Paul’s Epistles to the Thessalonians, p. 92.] Then He will be glorified "in the presence of" His saints (the locative use of the Greek preposition en). [Note: Wanamaker, pp. 230-31.] By using the Greek preposition en, Paul could have meant that Christ will be glorified both "among" them and "in" them.