Not what; Who. I gave you a one-sentence summary of the gospel in my previous post, so I'll stick with that answer.
If you were saved it would be WHAT as the gospel that is the power of God to save you is WHAT Christ died for/the WHY of the cross! 1 Corinthians 15:1-4 KJV
Because it was kept secret. The LORD from heaven did not appear until roughly 4BC.
Actually, it was kept secret until the risen Lord appeared to Paul (Acts 26:16 KJV). And that it would include us, another appearing and sending (Acts 22:17-21 KJV)
I can't speak to God's motives with certainty. My best guess... to make people seek Him.
Sure you can! We don't have to guess when God tells us why through the apostle Paul! This is why:
1 Corinthians 2:6 Howbeit we speak wisdom among them that are perfect: yet not the wisdom of this world, nor of the princes of this world, that come to nought:
1 Corinthians 2:7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:
1 Corinthians 2:8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
Among many places... Deuteronomy 32:21. Isaiah 65:1. Paul says so in Romans 10:19-20.
The NT quotes the OT constantly.
:nono: Sorry, but all of that does not apply to us. Where Gentiles such as we (Ephesians 2:11-12 KJV) are concerned it was a mystery/ which in other ages was not made known unto the sons of men/unsearchable/hid in God!
Ephesians 3:1 For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
Ephesians 3:2 If ye have heard of the dispensation of the grace of God which is given me to you-ward:
Ephesians 3:3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
Ephesians 3:4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
Ephesians 3:5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
Ephesians 3:6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Ephesians 3:7 Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power.
Ephesians 3:8 Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ;
Ephesians 3:9 And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:
Bless God He had a mystery that included the likes of us!
Bless God, we are in the but now (Romans 3:21-22 KJV)!
Ephesians 2:13 But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ.