1 Peter 3:19

It was by the Holy Spirit that He went to preach [See Eph. 2:17] to the [now departed] spirits [of pre-Flood people who are now] in prison [i.e., Hades].

1 Peter 4:6

This is the purpose that the Gospel was preached to those who have died physically [for the faith]: It was so that, even though they had been judged physically [i.e., by suffering death at the hands of men], they could still live spiritually in the presence of God.

1 Peter 1:11-12

They studied [their own prophecies] to learn what, or which, [person or] time the Holy Spirit in them pointed to, in predicting the sufferings of Christ and the glories yet to come.

Revelation 19:10

Then I fell down in front of him to worship him. But he said to me, "Do not do that; I am [only] a fellow-servant with you and with your brothers who hold onto the testimony about Jesus. You should worship God. For the testimony about Jesus is the spirit [i.e., the purpose, or source] of prophecy."

Revelation 20:7

And when the thousand years are over, Satan will be released from his prison,

Treasury of Scripture Knowledge

Summary

By which also he went and preached unto the spirits in prison;

Bible References

By which

1 Peter 1:11
They studied [their own prophecies] to learn what, or which, [person or] time the Holy Spirit in them pointed to, in predicting the sufferings of Christ and the glories yet to come.
1 Peter 4:6
This is the purpose that the Gospel was preached to those who have died physically [for the faith]: It was so that, even though they had been judged physically [i.e., by suffering death at the hands of men], they could still live spiritually in the presence of God.
Revelation 19:10
Then I fell down in front of him to worship him. But he said to me, "Do not do that; I am [only] a fellow-servant with you and with your brothers who hold onto the testimony about Jesus. You should worship God. For the testimony about Jesus is the spirit [i.e., the purpose, or source] of prophecy."

In

Revelation 20:7
And when the thousand years are over, Satan will be released from his prison,