1 Peter 4:13

καθὸ κοινωνεῖτε = in proportion as you have personal fellowship in the sufferings of the Christ. Christians are regarded not merely as suffering with (συμπάσχοντες) Christ, (Romans 8:17), but as members of His body they have a personal share in His sufferings, cf. Philippians 3:10; Colossians 1:24; 2 Corinthians 1:5. Suffering was the necessary prelude to glory in the case of Christ their Head, therefore His members can rejoice in present sufferings as being the prelude to glory in which they too will share when it is revealed. For rejoicing in suffering cf. Matthew 5:12; Luke 6:23; Acts 5:41; 2 Corinthians 6:10; Philippians 2:17; Colossians 1:24, etc.

χαίρετε ἵνα might possibly be explained as in John 8:56 “Your father Abraham ἠγαλλιάσατο ἵνα = rejoiced in the effort to see my day.” Abraham’s joy was that of anticipation and not that of present realization. So the joy of Christians in suffering is prompted by their anticipation of their exultation in the glory which is to follow. But it is simpler to regard joy in suffering as a preparation for the final joy. (See J. H. Moulton, Gram. pp. 205 ff.)

χαρῆτε�. ἀγαλλιᾶσθαι denotes exultant joy. Here such exultation is only regarded as possible when suffering culminates in glory, the joy during the process of suffering being of a more chastened character. But in 1 Peter 1:6; 1 Peter 1:8 ἀγαλλιᾶσθαι is used of the Christian’s present joy despite his griefs. The two words are combined in Matthew 5:12; Revelation 19:7.