Know the Unknown God
Paul was briefly alone in Athens, and he found this great city of pagan antiquity utterly given over to idolatrous worship. Consideration of the lost souls that were perishing in this city, and - worse - of the denial by these people of the rightful worship due to the true God, provoked Paul to speak out. God ought to be worshipped, and Paul proclaimed the gospel primarily that God should be glorified.
It was true of the people of this city that, “professing to be wise they became fools.” They spent their whole time in idle speculation. Paul’s response was to declare the unknown God to them, and to call them to worship.
So it is with us by nature: we are ignorant of the true God. In our fallen condition the truth that we know is not retained. The knowledge of God needs to be repeated, and meditated upon; our minds are sieves.
Like the Athenians, we are by nature idolatrous, denying the true God and worshipping creatures of our imagination. We are called in the gospel to know the unknown God.