"And God, that knoweth the hearts, gave testimony unto them:" he refers them to the spiritual testimony: "by giving them the Holy Ghost even as unto us." Everywhere he puts the Gentiles upon a thorough equality. Then he shows that the testimony given them is not of grace merely, but of their virtue. "And God which knoweth the hearts bare them witness;" having afforded to them nothing less than to us, for, he says, "Put no difference between us and them." Why then, hearts are what one must everywhere look to. And it is very appositely said, "God that knoweth the hearts bare them witness:" as in the former instance, "Thou, Lord, that knowest the hearts of all men." For to show that this is the meaning, observe what he adds, "Put no difference between us and them."
[AD 407] John Chrysostom on Acts 15:8