In the Old Testament, the concept of the afterlife was barely developed—neither heaven nor hell as we think of them today. Everyone, righteous or not, went to Sheol, a shadowy place for all the dead (Ps 88:48, Eccl 9:10). The idea of a distinct heaven or hell wasn’t clear because Jesus hadn’t yet made the ultimate sacrifice for believers’ sins (Heb 10:12). After His death, Jesus descended to the dead to lead the Old Testament righteous out (1 Pet 3:19–20), and the repentant thief on the cross was the first to enter paradise with Him (Luke 23:43). This redefined the afterlife.
Jesus spoke clearly about Gehenna (Matt 5:22, Mark 9:43) and explained the divide between the saved and the lost in the parable of Lazarus and the rich man (Luke 16:19–31). That seems pretty straightforward—He laid it out plainly. As for why the Apostles didn’t ask more, not every conversation was recorded (John 21:25), so they might’ve discussed it off-script.
Also, Jesus’ character aligns perfectly with the God of the Old Testament. Both are one in essence, showing justice, wrath against sin, mercy, and love. The Old Testament calls God a jealous Judge (Nah 1:2, Lev 26:14–33) yet merciful (Ex 34:6). Jesus judges (Matt 13:40–42, John 5:29), warns of wrath (John 3:36), and offers salvation (John 3:16). Revelation ties it together, showing Jesus as King and Judge (Rev 19:11–21), fulfilling the Old and New Testament’s God. So, the Apostles likely saw Jesus’ teachings as a natural extension of what they already knew about God’s justice and mercy.