Why did the Old Testament prophets search and inquire so diligently about salvation if they didn't understand it?
They searched diligently because the salvation they prophesied was not revealed to them. The Spirit of Christ within them testified beforehand of Christ's sufferings and glory, but they did not understand the Gospel of Grace. God had kept it secret, and they ministered these things not for themselves but for us.
Les explains that the Old Testament prophets inquired and searched diligently because they did not know it all; otherwise, why look and ask? They did not know, so they searched and inquired and prophesied of the grace that should come unto you. They were searching what, or what manner of time, the Spirit of Christ in them did signify when He testified beforehand the sufferings of Christ and the glory. It was revealed that not unto themselves, but unto us, they did minister the things now reported by those who preached the gospel, which the Holy Spirit sent down from heaven—things the angels desire to look into. Les emphasizes that they did not understand the Gospel of Grace; God never intended for them to understand it, for He had been keeping it secret. If they had understood it, and if Abraham had seen that Christ would come, die, be raised, and offer salvation to the whole world, then the verse in Romans 16 would be almost spurious, but it is the truth.
Of which salvation the prophets have enquired and searched diligently, who prophesied of the grace that should come unto you:
Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow.
More Questions
Why could the Old Testament prophets not understand their own prophecies?
The Old Testament prophets could not put it all together—they did not fully understand how the Messiah's suffering and the coming glory fit together.
What were the Old Testament prophets searching for in 1 Peter 1?
The Old Testament prophets were not simply telling the future; they were also searching to learn what or what manner of time the Spirit of Christ in them was indicating as they wrote.