Charles Ellicott Commentary 2 Thessalonians 1:10

"when he shall come to be glorified in his saints, and to be marvelled at in all them that believed (because our testimony unto you was believed) in that day." — 2 Thessalonians 1:10 (ASV)

When he shall come — This is not simply a repetition of the temporal date mentioned in 2 Thessalonians 1:7—“when the Lord,” and so on—but an introduction of the contrast that will be presented “in that day” by the spectacle of the glory of the saints. Thus, the penalty of 2 Thessalonians 1:9 is shown to be even greater, while at the same time the readers’ minds are turned back to a more wholesome subject for meditation.

To be glorified in his saints — This is not exactly the purpose, but the effect of His coming. A comparison of John 13:31-32, John 14:13, John 17:10, and 2 Thessalonians 1:12 shows that the saints are the objects on which and by which the glorious perfection of Christ is exhibited. To see what the saints will be exalted to “in that day” will make all observers acknowledge not the holiness or greatness of men, but the divine power of Him who was able to exalt them so. As the persecutors were divided into two classes to be punished, so the saved are described under two aspects: in contrast with them that know not God, they are “saints” (that is, fully consecrated to God); in contrast with them that obey not the gospel, they are they that believed (for the past tense is the better reading), that is, they accepted the gospel. As the profane Gentiles, looking on the saints, recognize the “glory” of the God whom they knew not, so the disobedient Jews, seeing the faithful, are aptly filled with wonder (Acts 13:41) before they perish, at the glory to be attained by obedience to the law of suffering.

Because our testimony — This phrase is introduced to show why the writers specifically said in all them that believed (the past tense is used because it looks back from the Judgment Day to the moment the gospel was offered and the divergence between believers and unbelievers began). The reason was that the Thessalonians would be found included among all them that believed.

In that day — This is added at the end to make the readers look once more, as it were, at the wonderful sight on which the writer’s prophetic eyes were raptly fixed.