Why couldn't Old Testament believers go to heaven when they died?
Old Testament believers could not enter glory because their sins were not atoned for but only covered. Animal blood could not take away sin; it merely covered it under Kephar/atonement until the true atoning work of the cross, after which sins were atoned and paradise was taken into heaven.
Les explains that atonement is at-one-ment, but no man can become one with God until he has experienced the resurrection power and the finished work of the cross, which had not yet happened. The blood of animals could not take away sin; it could only cover it. The Hebrew word translated Atonement was Kephar, meaning to cover, as when something is covered with bitumen. So the Day of Atonement was not a day of removing Israel's sin, but a covering. David in the Psalms said, "Blessed is the man whose sins are covered." They were not done away with. That is why the Old Testament believer could not go into glory when he died: his sins were not atoned for, they were covered. They were waiting for the true atoning work of the cross. After the cross, after the supreme sacrifice, the divine blood had been shed, their sins were atoned for, and God took paradise up into heaven.