When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed) in that day.
'When He shall have come.'
Glorified in his saints - as the element and mirror IN which His glory shall shine brightly; not, as in 1 Thessalonians 3:13, including angels, but only saints (Isaiah 49:3), which involves their being glorified (John 17:10; John 17:22).
Admired in all them that believed - Greek, 'them that believed.' Once they believed; now they see. They took His word on trust; now His word is made good, and they need faith no longer. With wonder all celestial intelligences (Ephesians 3:10) shall admire the Redeemer, because of the glory He has worked in them that believed during their earthly probation.
Because ... Supply 'among whom (namely, those who shall be found to have believed) you, too, shall be; because our testimony (1 Corinthians 1:6) unto х epi (G1909): "among"] you was believed,' and not rejected, as by those who "obey not the Gospel" (2 Thessalonians 1:8). The early preaching was not abstract discussions, but a testimony to facts and truths experimentally known (Luke 24:48; Acts 1:8). Faith is (Dr. Pearson) 'an assent unto truths, credible upon the testimony of God, delivered unto us by the apostles and prophets' (at first orally, but now in their writings). "Glorified in His saints" reminds us that holiness is glory in the bud; glory is holiness developed.