Jamieson, Fausset and Brown on 1 Peter 1:6

6. Wherein--in which prospect of final salvation.

greatly rejoice--"exult with joy": "are exuberantly glad." Salvation is realized by faith (1Pe 1:9) as a thing so actually present as to cause exulting joy in spite of existing afflictions.

for a season--Greek, "for a little time."

if need be--"if it be God's will that it should be so" [Alford], for not all believers are afflicted. One need not invite or lay a cross on himself, but only "take up" the cross which God imposes ("his cross"); 2Ti 3:12 is not to be pressed too far. Not every believer, nor every sinner, is tried with afflictions [Theophylact]. Some falsely think that notwithstanding our forgiveness in Christ, a kind of atonement, or expiation by suffering, is needed.

ye are in heaviness--Greek, "ye were grieved." The "grieved" is regarded as past, the "exulting joy" present. Because the realized joy of the coming salvation makes the present grief seem as a thing of the past. At the first shock of affliction ye were grieved, but now by anticipation ye rejoice, regarding the present grief as past.

through--Greek, "IN": the element in which the grief has place.

manifold--many and of various kinds (1Pe 4:12, 13).

temptations--"trials" testing your faith.