Who were the Old Testament prophets ministering to when they prophesied about Christ?
The Old Testament prophets, when they prophesied about Christ, were not ministering to themselves but to us—the ones who would later receive the gospel. They searched diligently, yet it was revealed that their ministry was for those who would hear the gospel preached by the Holy Spirit sent from heaven.
Les explains that the Old Testament prophets did not fully understand the Gospel of Grace. They inquired and searched diligently about the sufferings of Christ and the glory that would follow, but God had kept it secret. As Peter writes, it was revealed to them that they were not serving themselves but us, the ones who would later have the gospel reported to them by those who preached it through the Holy Spirit sent from heaven. Even angels desire to look into these things. Les ties this to Romans 16, noting that if the prophets had understood, the verse would be spurious, but it is the truth.
Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Ghost sent down from heaven; which things the angels desire to look into.