You are confusing the almightiness of God with His own conditions for salvation. God compels no one to be saved, but He does present the consequences of unbelief to all (Mark 16:15,16).
Even though God desires the salvation of all mankind (1 Tim 2:4), He will not save anyone who fails to obey the Gospel, which is called "the obedience of faith" (Rom 16:25,26). Which means (1) repentance towards God and (2) faith towards our Lord Jesus Christ.
And that is exactly why Paul said this is Acts 20:18-21:
18 And when they were come to him, he [Paul] said unto them, Ye know, from the first day that I came into Asia, after what manner I have been with you at all seasons,
19 Serving the Lord with all humility of mind, and with many tears, and temptations, which befell me by the lying in wait of the Jews:
20 And how I kept back nothing that was profitable unto you, but have shewed you, and have taught you publickly, and from house to house,
21 Testifying both to the Jews, and also to the Greeks, repentance toward God, and faith toward our Lord Jesus Christ.
God predestines no one for salvation or damnation. And that is why these words of Paul are critical to understanding how sinners are saved by grace through faith.