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.
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Ambrosiaster
AD 400
in that day
The Gospel will bear witness to them in the Day of the Lord.
This shall be at the last day, when Christ shall come to be glorified in his saints, and to be made was believed upon you concerning that day, i.e. as to the punishments and rewards that shall be given on that day. (Witham)
Is God glorified? Yea, he says, in all the Saints. How? For when they that puff so greatly see those who were scourged by them, who were despised, who were derided, even those now near to Him, it is His glory, or rather it is their glory, both theirs and His; His indeed, because He did not forsake them; theirs, because they were thought worthy of so great honor. For as it is His riches, that there are faithful men, so also it is His glory that there are those who are to enjoy His blessings. It is the glory of Him that is good, to have those to whom He may impart His beneficence. And to be marveled at, he says, in all them that believed, that is, through them that believed. See here again, in is used for through. For through them He is shown to be admirable, when He brings to so much splendor those who were pitiable and wretched, and who had suffered unnumbered ills, and had believed. His power is shown then; because although they seem to be deserted here, yet nevertheless they there en...
because our testimony has been believed by you
They also call their testimony evangelical teaching, that which indeed was entrusted to them, because they received evangelical teaching from those preaching it.