The "mystery" was in regard to this prophesied "grace" which was promised to the Israelites, and it concerned what the suffering of the Lord did signify:
"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. 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 Spirit sent down from heaven" (1 Pet.1:10-1).
Here Peter is saying that the prophets searched diligently in an effort to determine what the prophecies concerning Christ's suffering did signify but it was not revealed unto them. Even the Twelve Apostles, those closest to the Lord Jesus, did not realize that He was going to die (Lk.18:31-34) or be resurrected (Jn.20:9) until shortly before the Cross. They certainly did not know the "purpose" of the Cross, that believers are "justified freely by his grace through
the redemption that is in Christ Jesus" (Jn.3:24).
Roger M. Raymer writes:
"Concerning this salvation (cf. 'salvation' in vv. 5, 9) the prophets...searched intently and with the greatest care their own Spirit-guided writings. They longed to participate in this salvation and coming period of grace and tried to discover the appointed time and circumstances to which the Spirit of Christ in them was pointing. The pondered how the glorious Messiah could be involved in suffering" (Walvoord & Zuck, The Bible Knowledge Commentary; New Testament [ChariotVictor Publishing, 1983], p.842).