Albert Barnes on 1 Peter 4:13

But rejoice, inasmuch as ye are partakers of Christ's sufferings. That is, sufferings of the same kind that he endured, and inflicted for the same reasons. Comp. Col 1:24, Jas 1:2; Mt 5:12. The meaning here is, that they were to regard it as a matter of rejoicing that they were identified with Christ, even in suffering. See this sentiment illustrated at length Php 3:10.

That, when his glory shall be revealed. At the day of judgment. Mt 26:30. Ye may be glad also with exceeding joy. Being admitted to the rewards which he will then confer on his people. Comp. 1Thes 2:19. Every good man will have joy when, immediately at death, he is received into the presence of his Saviour; but his joy will be complete only when, in the presence of assembled worlds, he shall hear the sentence which shah confirm him in happiness for ever.

(d) "rejoice" Jas 1:2 (e) "when his glory" 2Ti 2:12