‘When he shall come to be glorified in his saints, and to be marvelled at in all those who believed, (because our testimony to you was believed), in that day.'
‘That day' is a technical term used to designate the day of the appearing of our Lord Jesus Christ (2 Timothy 1:12; 2 Timothy 1:18; 2 Timothy 4:8; Matthew 17:22; Matthew 24:36; Matthew 26:29; Mark 13:32; Mark 14:25; Luke 17:31; Luke 21:34). It is the day when He is glorified by the transformation for ever of His own people, His separated ones (saints), the day when the universe and the angels will marvel at what He has done for them and in them, and what they have become (see Ephesians 3:10), will also be the day when the wicked are severed from among the just and are destroyed for ever from before His face. The contrast is huge. On the one hand splendour and glory given to His people by the Lord Himself, on the other eternal loss and ruin dispensed to the unbelievers and the disobedient. And the Thessalonians would share that splendour and glory because they had believed the message that Paul and his companions preached, and had received their testimony.
‘Marvelled at in all those who believed.' Compare Revelation 5:9-10; Revelation 5:12; Psalms 118:22-23. Some, however, would see it as meaning that those who believed will marvel at Him and His wondrous works, but the parallel suggests that the marvelling is from an external source.