Complex sentence: God is not the cause of sin since, on the contrary, he draws all things to himself; but from another side, God is the cause of all things, so he is efficacious also in sin as actio "but not as" ens.
Split into simpler sentences: God is not the cause of sin since, on the contrary, he draws all things to himself. But from another side God is the cause of all things, so he is efficacious also in sin as actio "but not as" ens.