Why did the doctrine of life after death have to begin with Christ's resurrection?
Christ's resurrection was necessary because all prior raisings, like Lazarus, were merely restorations to physical life that ended in death again. True resurrection commenced when the Creator Himself was raised, thereby opening the door for the entire doctrine of life after death.
Les explains that resurrections such as Elijah raising the widow's son, or Jesus raising Lazarus, were not resurrection; they were merely bringing someone back to life only to die again. Resurrection never took place until Jesus was raised from the dead. Christ's own resurrection opened the door to the whole doctrine of life after death, and it had to begin with the Creator Himself. Immediately after His resurrection, to show its power and fulfill first fruits as practiced in the Old Testament economy, graves were opened in the Jerusalem area, and those raised went into the Holy City and appeared to many. Les notes Scripture does not say they went on up into glory, but they had to have because they were resurrected, not merely raised to die again. This fulfilled the whole idea of first fruits.