Paul said,

“But as it is written:
‘Eye has not seen, nor ear heard,
Nor have entered into the heart of man
The things which God has prepared for those who love Him.” (1 Cor 2:9)
Understandably, people often think of heaven when they read this verse. After all, heaven is a place that is prepared, and people who love God will be in it (John 14:1-4; 2 Tim 4:8). But that’s not what this verse is about. If this isn’t talking about heaven being so great that we couldn’t imagine it, then what is it talking about?
Something greater. The mystery of God.
We can immediately clarify most verses in the Bible by reading what is written before and after them. In this case, Paul had just finished saying that he spoke the wisdom of God in a mystery that was hidden. If the rulers of the time had known the mystery, they would not have crucified Jesus (1 Cor 2:8-9). The mystery is what no one could have imagined. The mystery is what God prepared for those who love Him. While heaven is part of it, it is not the part Paul was talking about. His focus was on what makes heaven possible.
How do we know? Because the mystery is no longer unknown. Paul was writing through the inspiration of the Holy Spirit (2 Tim 3:16-17; 2 Pet 1:20-21). The Spirit Who searches the deep things of God let Paul know what the mystery was. That was what Paul was teaching and writing (1 Cor 2:9-13).
Then what was he writing about? He was preaching and writing what he always had since he converted from Judaism to Christianity (Acts 9). The gospel, the message of the cross, is the power of God for salvation (1 Cor 1:18; Rom 1:16). Before He spoke the universe into existence, the Lord planned to send His Son to earth to die for us, so that all who obey the gospel can have the forgiveness of sins (Eph 1:1-10; Mark 16:15-16; Acts 2:22-41).
The mystery that was revealed here and there through time has now been entirely shared in the Word of God. God made it possible for all people to be heirs of heaven through the same body – one church – in Christ (Eph 1:22-23; 3:1-7; 4:1-6). That is the church for which Jesus died and will come again to take to heaven (Eph 5:25-27).
“And without controversy great is the mystery of godliness:
God was manifested in the flesh,
Justified in the Spirit,
Seen by angels,
Preached among the Gentiles,
Believed on in the world,
Received up in glory”
(1 Tim 3:16).
-Van