Why didn't Abraham and the Old Testament saints understand the death, burial, and resurrection of Christ and His atoning blood?
Abraham and the Old Testament saints understood resurrection and Israel's redemption, but could not associate these with Christ's death, burial, resurrection, and atoning blood. The context says they had no concept of that because God did not expect them to die, so that truth was not part of their understanding.
Les explains that Abraham had the concept of resurrection and Israel coming into a place of redemption on his mind, but he and the Old Testament saints could not associate redemption with a death, burial, and resurrection, or with the shedding of Christ's atoning blood. The stated reason is that God did not expect them to die. Les also notes that Abraham was the man of faith and the friend of God, and that believers today are blessed the same faith way plus nothing, because of what was accomplished in the work of the cross.