There's this idea that before Jesus, everyone - saved or unsaved- went to Hades, basically you'd just rot forever, 'saved or not'. After Jesus' death & resurrection, those who were righteous, their spirits went to heaven.
I do not know what to think about that.
The emphasis is usually a lot in the Old Testament are saved. We do know that. Hebrews describes a great cloud of witnesses who are examples of the faith, including OT prophets. Abel is mentioned, not Cain. Adam is not mentioned, but few Christians will doubt Adam wasn't saved. Moses & Elijah also appear at the Transfiguration.
(I wonder if their appearance at the Transfiguration dismisses the Hades idea???)