1 Peter 4:5-6

⁵ They will give an account to Him who is ready to judge the living and the dead.

⁶ For this reason the gospel was preached also to those who are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.

The word [“they”] (v.5) is in apposition with the participle translated “speaking evil,” and refers to the people of the world who shall give an account to God who is holding Himself in readiness (Greek) to judge the living and the dead. — Wuest, page 113.

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The key to the understanding of this difficult verse is found in the context of the entire book. In 1:6-7 we are told that the recipients are in heaviness in the midst of manifold trials. In 2:18-25 we have the case of Christian household slaves being unjustly punished because of their Christian testimony. In 3:8-17 the saints are instructed as to their behavior when undergoing persecution. In 4:12-19 the apostle deals with the glory of suffering for righteousness’s sake. He speaks of this persecution of the saints by the world as a judgment that begins at the house of God (4:17). In 4:1-11 he speaks of the necessity of having the mind of Christ as armored protection against the persecution of the world. Thus the phrase “judged according to men,” refers to the judgment spoken of in 4:17 which is defined as to its nature by the words in verse 14, “If ye be reproached for the name of Christ.”

The words “them that are dead” refer to believers who had died. … [Their] judgment was in the form of persecution because of their testimony. — Wuest, pages 113-114.

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The murder of God’s servants was not accidental, or because the Devil was stronger than God, but because God had a purpose—hidden from His people but satisfactory to Himself—in permitting such cruelty. This permission manifested both the malignity of man’s nature against God, and the triumph of faith in God. The blessed martyrs (the “dead”) were judged by men to be unfit for human society, and were consequently killed, but they were judged by God to be worthy of heaven’s society and consequently glorified. — Williams, pages 1002-1003.

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Peter deals with the judgment of the unsaved living and dead when he states: “Who shall give account to Him that is ready to judge the quick and the dead.” the “unsaved living” who are alive at the Second Coming of Christ [none of whom will be members of the Body of Christ because they will have been raptured before the Tribulation] will be judged at the beginning of the kingdom and cast into outer darkness (Ezekiel 20:33-38; Matthew 13:36-43; 25:31-33, 41-46). The “unsaved dead” of all ages will be judged at the Great White Throne Judgment (Revelation 20:11-15). — Sadler, page 137.

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The chapters and passages leading up to this particular verse [v.6] have been dealing with suffering, especially in relation to the unjust treatment of these Jewish believers. In fact, some had experienced unspeakable atrocities … There is little question at this juncture that Peter is speaking about believers. The gospel had at one time been preached to them, they believed, and were called upon to suffer and be rejected  like their Savior. As victims of miscarriages of justice, these saints were falsely accused and judged according to man’s judgment. Thus, because of their testimony, some had suffered martyrdom for their faith. but death was not the end, even though the silence of the grave gave them impression. These saints were alive and well and serving the Lord in a disembodied state. — Sadler, pages 137-138.

So my understanding of verse 6 is that those believing Jews who had died for their faith in the ongoing persecution (and who will die for their faith in the Tribulation), were judged worthy by God and were alive in Him in their spirits even though they had been judged unworthy by men and put to death in the flesh.