Why couldn't Old Testament saints understand the mystery of the gospel?
Old Testament saints could not understand the mystery of the gospel because God kept it secret in His own mind and did not reveal it to them. There was no way He would let them understand it; the mystery remained hidden by His sovereign purpose until He chose to open their eyes.
Les explains that the Old Testament saints couldn't know the mystery because it was a secret kept in the mind of God, and God was not going to let them understand it. The Twelve also didn't understand the prophetic references to His death and resurrection. They had no idea He was going to the Roman cross, and after He hung there they did not expect to see Him on Sunday. They thought it was all over; Peter said, "I go fishing," because everything seemed hopeless. God kept it secret and did not reveal it to people. Les sees the same principle today: many are kept blind to the truth of the Gospel until God opens their eyes, and then they become intensely responsible. The Gospel preached unto Abraham was not a Gospel Abraham understood; he only had a concept that God was going to send a Redeemer.