1 Peter 4:6
4 So they are astonished when you do not rush with them into the same flood of wickedness, and they vilify you. 5 They will face a reckoning before Jesus Christ who stands ready to judge the living and the dead. 6 Now it was for this very purpose that the gospel was preached to those who are now dead, so that though they were judged in the flesh by human standards they may live spiritually by God’s standards. 7 For the culmination of all things is near. So be self-controlled and sober-minded for the sake of prayer. 8 Above all keep your love for one another fervent because love covers a multitude of sins.
Irenaeus Against Heresies 4.27.2
2 It was for this reason, too, that the Lord descended into the regions beneath the earth, preaching His advent there also, and [declaring] the remission of sins received by those who believe in Him. Now all those believed in Him who had hope towards Him, that is, those who proclaimed His advent, and submitted to His dispensations, the righteous men, the prophets, and the patriarchs, to whom He remitted sins in the same way as He did to us, which sins we should not lay to their charge, if we would not despise the grace of God. For as these men did not impute unto us (the Gentiles) our transgressions, which we wrought before Christ was manifested among us, so also it is not right that we should lay blame upon those who sinned before Christ's coming. For all men come short of the glory of God, and are not justified of themselves, but by the advent of the Lord, — they who earnestly direct their eyes towards His light.
Notes and References
"... 1 Peter 4:6 uses the passive voice to describe proclaiming the gospel to the dead (thus making the agent of the preaching unclear). There are two ways to interpret this proclamation. (1) Those who demonstrated faith in God prior to the advent of Jesus are now saved through the same gospel so that they too might live in the spirit (Horrell 2003; compare Hebrews 11:39–40). (2) Those who accepted the gospel but have passed away before Jesus’ glorious return still live in the spirit (Dalton 1965; compare 1 Thessalonians 4:13–18) ..."
Gregg, Brian Han "1 Peter" in Aune, David Edward, (ed.) The Blackwell Companion to the New Testament (p. 591) Wiley-Blackwell, 2010