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 come to be glorified in his saints - As the grace of God is peculiarly glorified in saving sinners and making them into saints, this gracious power will be particularly manifested in the great day, when countless millions will appear before that throne who have come out of great tribulation, and have washed their robes and made them white in the blood of the Lamb.
And to be admired - Οαυμασθηναι· To be wondered at among and on the account of all them that believe. Much as true believers admire the perfections of the Redeemer of mankind, and much as they wonder at his amazing condescension in becoming man, and dying for the sins of the world; all their present amazement and wonder will be as nothing when compared with what they shall feel when they come to see him in all his glory, the glory that he had with the father before the world was. In reference to this we may apply those words of St. John: "Beloved, now are we the sons of God; and it doth not yet appear what we shall be: but we know that when he shall appear we shall be like him; for we shall see him as he is." 1 John 3:2.
Instead of τοις πιστευουσιν, them that believe, τοις πιστευσασιν, them that have believed, is the reading of ABCDEF, many others, the later Syriac, Slavonic, Vulgate, and Itala, with most of the Greek fathers. This reading is undoubtedly genuine.
Because our testimony - was believed in that day - The members of this sentence seem to have been strangely transposed. I believe it should be read thus: "In that day, when he shall come to be glorified in his saints, and admired among all them that have believed; for our testimony was believed among you." The Thessalonians had credited what the apostles had said and written, not only concerning Jesus Christ in general, but concerning the day of judgment in particular.