Descent into Hell, Tintoretto, 1568
Today in the Church calendar is Holy Saturday, the day when we remember Christ's descent into hell.
Every Sunday, Christians all over the world confess their belief in Christ's descent into hell when they recite The Apostles' Creed. This doctrine is based upon 1 Peter 3:19 and 4:6.
C. S. Lewis had this to say about the doctrine in a letter to Mary Van Deusen, written on January 31, 1952. Mrs. Van Deusen must have asked Lewis about the fate of the "heathen" for Lewis answers...
On the heathen, see I Tim. IV. 10. Also in Matt. XXV. 31-46 the people don't sound as if they were believers. Also the doctrine of Christ's descending into Hell and preaching to the dead: that would be outside time, and include those who died long after Him as well as those who died before He was born as Man. I don't think we know the details: we must just stick to the view that (a.) All justice & mercy will be done, (b) But that nevertheless it is our duty to do all we can to convert unbelievers.