Will Duffy writes an article: How the Salvation of Cornelius Refutes Calvinism.
Monergism would necessitate that Cornelius has already been regenerated at this point. For Calvinism would not describe an unsaved man the way Luke describes Cornelius here. Yet prior to his conversion, Cornelius was a devout man, and feared God. He also gave alms and prayed to God always. Though contradicted by numerous biblical examples, according to Calvinism, unregenerate men cannot do good in the sight of God. And in contrast to Ezekiel’s warning to the man whose “righteousness which he has done shall not be remembered”, regarding Cornelius, the Apostle Peter says, “Cornelius, your prayer has been heard, and your alms are remembered in the sight of God” (Acts 10:31). Yet the book of Acts shows clearly that Cornelius had yet to experience salvation.