The fact remains that some things were
necessarily "kept secret/hidden" until AFTER Jesus' crucifixion.
[1Cor2:7-8]
In view of that, let me just post a paragraph by William Kelly on this particular topic ^ :
[quoting]
"We must, however, guard against the notion that "the mystery" or secret means
the gospel. The gospel in itself does not and never can mean a mystery. It was that which in its foundations always was before the mind of God's people in the form of promise, or of a revelation of grace not yet accomplished. But nowhere in Scripture is the gospel called a mystery. It may be connected with the mystery, but it is not itself a mystery. It was no mystery that a Saviour was to be given; it was the very first revelation of grace after man became a sinner. The Seed of the woman was to bruise the serpent's head.
A mystery is something that was not revealed of old, and which could not be known otherwise. Again, you have in the prophets a full declaration that the righteousness of God was near to come; the plainest possible statement that God was going to show Himself a Saviour-God. So again you have His making an end of sins and bringing in reconciliation and everlasting righteousness.
All these things were in no sense the mystery. The mystery means that which was kept secret, not that which could not be understood, which is a human notion of mystery; but an unrevealed secret, - a secret not yet divulged in the Old Testament but brought out fully in the New. What, then, is this mystery? It is, first, that Christ, instead of taking the kingdom, predicted by the prophets, should completely disappear from the scene of this world, and that God should set Him up in heaven at His own right hand as the
Head of all glory, heavenly and earthly, and that He should give the whole universe into the hands of Christ to administer the kingdom and maintain the glory of God the Father in it. This is the first and most essential part of the mystery, the second, or Church's part, being but the consequence of it. Christ's
universal headship is not the theme spoken of in the Old Testament. You have Him as Son of David, Son of man, Son of God, the King; but nowhere is the whole universe of God (but rather the kingdom under the whole heavens) put under Him. In this headship over all things, Christ will share all with His bride. Christ will have His Church the partner of His own unlimited dominion, when that day of glory dawns upon the world.
Hence, then, as we know, the mystery consists of two great parts, which we have summed up in
Ephesians 5:32; "This is a great mystery; but I speak concerning Christ and the church." "
[end quoting; bold and underline mine]