It had been revealed to him by the Holy Spirit that he would not see death before he had seen the Christ of the Lord.
In context
24They also came to offer the sacrifice named in the law of the Lord: a pair of turtledoves or two young pigeons. 25Now there was a man in Jerusalem named Simeon. He was upright and devout, waiting for the consolation of Israel, and the Holy Spirit was upon him. 26It had been revealed to him by the Holy Spirit that he would not see death before he had seen the Christ of the Lord. 27Moved by the Spirit he came into the temple, and when the parents brought in the child Jesus to do for him what the law required, 28Simeon took him into his arms, blessed God and said: