These were not Jews. Nor were they people with a background in Judaism or philosophy. As they were revealing they were out and out idolaters and simple men. But Paul does not turn away from them. Rather he speaks to them in a way that they will understand. Of no use here is it to mention the past history of the Jews or Greek philosophers. So he proclaims the past history of the world, although in Old Testament terms, so as to draw out that there is only one God, and that He is calling all men now to respond to Him.
He points away from himself and Barnabas, who are but men of similar desires to them, to the Creator of heaven and earth and of all that is (compare Acts 4:24; Psalms 146:6 LXX. See also Nehemiah 9:6; Isaiah 37:16; Psalms 69:34). In the past He had left men to walk in their own ways (Isaiah 53:6 LXX - although having provided them with a conscience, a law within - Romans 2:14-16). Yet even so He did not leave Himself without a witness in that He dispensed from heaven rains and fruitful seasons (Leviticus 26:4; Isaiah 55:10; Matthew 5:45) filling their hearts with food and gladness (compare Psalms 145:15-16 LXX). Thus they should see His power and compassion (His eternal power and Godhead - Romans 1:20) and turn from ‘vain things' (Jeremiah 2:5 LXX; Acts 14:22) to the living God (compare 1 Thessalonians 1:9) Who alone could do such things, turning away from the follies and vain things which were so clearly a constituent of idolatry.