What is the meaning of Acts 17:22-23?

22 Then Paul stood in the midst of Mars’ hill, and said, Ye men of Athens, I perceive that in all things ye are too superstitious. 23 For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you. (Acts 17:22-23 KJV)

William Burkitt’s Commentary

As if the apostle had said, “Ye men of Athens have a great number of gods, whom ye ignorantly worship: the God, therefore, whom ye acknowledge not to know, and yet profess to worship, is he that I preach unto you; for as I passed up and down in your city, beholding your altars and images, I found an altar with this inscription, TO THE UNKNOWN GOD.”

Here observe, 1. The light of nature discovered: the altar is inscribed, TO A GOD. The true God of the Jews was an unknown and uncertain God to the wisest of the Gentiles.

Learn, That some discoveries of God may be made even by the light of nature: these heathens who had nothing but the dim light of nature to guide and direct them, do yet own a God, and acknowledge that worship is due him, by the erection of an altar.

Observe, 2. The darkness of nature declared: the altar, though erected to a God, yet it’s to a God unknown.

Thence learn, That natural light, in its most elevated and raised improvements, can make no full and saving discoveries of God. The true God was but an unknown God, even to the wisest of the heathens, to the men of Athens, who were the most famous, in their day, for the severest wisdom and gravity.