Albert Barnes Commentary 1 Peter 4:13

"but insomuch as ye are partakers of Christ`s sufferings, rejoice; that at the revelation of his glory also ye may rejoice with exceeding joy." — 1 Peter 4:13 (ASV)

But rejoice, insofar as you are partakers of Christ's sufferings. This means sufferings of the same kind that He endured, and inflicted for the same reasons. (James 1:2).

The meaning here is that they were to regard it as a matter of rejoicing that they were identified with Christ, even in suffering.

That, when His glory will be revealed. This refers to the day of judgment.

You may be glad also with exceeding joy. This means being admitted to the rewards which He will then confer on His people. (Compare to 1 Thessalonians 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 will hear the sentence which will confirm him in happiness forever.