1 Peter 3:19

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

The spirits in prison were the living people of Noah's generation, shut up in the prison-house of bondage to sin, not disembodied spirits preached to after death. It was Christ Himself who preached to them, appealing to them while they lived through preaching by a human agent on earth. Those faithful witnesses included Seth and Enoch as well as Noah.

That generation was granted 120 years of probation in which to heed the warning, repent, and return to loyalty to God. Christ stood to them as their atoning sacrifice and mediator, yet His intercession did not avail for them because they had no faith. This preaching was part of God's constant appeal in every age for the human heart to recognize His love and mercy and to accept His righteousness in place of the principles of evil. The same Christ who preached in Noah's day afterward came to Israel enshrouded in a pillar of cloud by day and a pillar of fire by night, and with infinite wisdom educated that vast multitude through the wilderness wandering, teaching them daily what God requires His church on earth to be.

The verse bears a message of great importance to ministers of the gospel and to ministers' wives. It calls them to search deep and make clean work for eternity for the benefit of their own souls. It calls them to let the convicting, converting power of the gospel search their hearts and their families until no irreligious element remains in the home. It calls them to labor with zeal for unsanctified, unconverted companions in the home and in the churches.