"Amen, amen, I say to you,
the hour is coming and is now here
when the dead will hear the voice of the Son of God,
and those who hear will live." (Jn 5:25)
After his death, Christ's soul, descended to Sheol to liberate those who lived in the shadow of death. Where this place was is unknown except to say that it constituted a different dimension of the universe where the just who had died since the beginning of life on earth (possibly the beginning of life in the universe) lived.
St. Peter tells us that the gospel was preached to the dead (Pt 4:6), that is, He made known to them that He was God's Son. To the Jews in Sheol Christ was the expected Savior and to holy pagans Jesus was the revelation longed for while they lived on earth. Rejoicing was heard throughout Sheol and at the same time the powers of death that held them bound were defeated.
The starting point of the Resurrection was Sheol. In the icon above we see Christ leading Adam out while Eve waits. These two represented mankind. In the background most likely stands St. John the Baptist representing the Jews
For many years in the Apostles Creed we professed our belief that Christ "descended into hell". Since for many people this meant real hell, it has now being changed to read "descended to the dead". This still does not make clear the nature of Sheol but is an improvement.