"And put no difference between us and them, having purified their hearts by faith." From faith alone, he says, they obtained the same gifts. This is also meant as a lesson to those objectors; this is able to teach even them that faith only is needed, not works nor circumcision. For indeed they do not say all this only by way of apology for the Gentiles, but to teach the Jewish believers also to abandon the Law. When he has mentioned the testimony borne to them, then he utters that great word, the same which Paul speaks, "Neither circumcision availeth anything, nor uncircumcision." "That he may make the twain one in Himself." Of all these the seeds lie in Peter's discourse. And he does not say between them of the circumcision, but "Between us," that is the Apostles, "and them." Then, that the expression, "no difference" may not seem an outrage, After faith, he says-"Having purified their hearts by faith"-He thoroughly cleansed them first.
Faith in God purifies the heart, the pure heart sees God. But faith is sometimes defined as followed by people who wish to deceive themselves; as if it were enough merely to believe—some people, you see, promise themselves the vision of God and the kingdom of heaven for believing while living bad lives. Against these the apostle James indignantly took umbrage out of spiritual charity, so he says in his letter, “You believe that God is one.” You pat yourself on your back for your faith; you observe that many godless people assume there are many gods, and you congratulate yourself for believing that there is only one God. “You do well. The demons also believe—and shudder.” Shall they too see God? Those who are pure of heart shall see him. Whoever would say that the unclean spirits are pure of heart? And yet, “they believe—and shudder.”So our faith must be distinguished from the faith of demons. Our faith, you see, purifies the heart, their faith makes them guilty.… So let us distinguish our faith and see that believing is not enough. That is not the sort of faith that purifies the heart. “Purifying their hearts,” it says, “by faith.” But which faith, what sort of faith? The one, surely, which the apostle Paul defines when he says “faith that works through love.” This faith is different from the faith of demons, different from the morals of dissolute and desperate people. “Faith,” he says. “Which faith?” The one “that works through love,” hopes for what God promises. You could not have a more perfect, a more carefully thought-out definition than that.
Purifying their hearts by faith. Therefore, it is not necessary to cleanse them with the circumcision of the flesh, whose hearts such great faith purifies, that they even deserve to receive the Holy Spirit before baptism.
[AD 407] John Chrysostom on Acts 15:9