A.T. Robertson 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 (οταν ελθη). Second aorist active subjunctive with οταν, future and indefinite temporal clause (Robertson, Grammar, pp. 971ff.) coincident with εν τη αποκαλυψε in verse 7.

To be glorified (ενδοξασθηνα). First aorist passive infinitive (purpose) of ενδοξαζω, late verb, in N.T. only here and verse 12, in LXX and papyri.

In his saints (εν τοις αγιοις αυτου). The sphere in which Christ will find his glory at the Revelation.

And to be marvelled at (κα θαυμασθηνα). First aorist passive infinitive (purpose), common verb θαυμαζω.

That believed (τοις πιστευσασιν). Why aorist active participle instead of present active πιστευουσιν (that believe)? Frame thinks that Paul thus reassures those who believed his message when there (1 Thessalonians 1:6ff.; 2:13f.). The parenthetical clause, though difficult, falls in with this idea:

Because our testimony unto you was believed (οτ επιστευθη το μαρτυριον ημων εφ' υμας). Moffatt calls it an anti-climax.

On that day (εν τη ημερα εκεινη). The day of Christ's coming (2 Timothy 1:12,18; 4:8).