What had the Holy Spirit revealed to Simeon?

Answer: He'd see the Christ before dying

The Spirit revealed to righteous Simeon that he would not see death before he had seen the Lord's Christ.

“Now there was a man in Jerusalem named Simeon, who was righteous and devout. He was waiting for the consolation of Israel, and the Holy Spirit was upon him. The Holy Spirit had revealed to him that he would not see death before he had seen the Lord’s Christ.”— Luke 2:25-26 (BSB)