William Perkins:
Here observe a notable practice of Abraham, as a good direction how we ought to judge of all those that live in the church, submitting themselves outwardly to the ministry and regiment thereof. Abraham here hath two sons, Isaac and Ishmael; he circumciseth them both, and instructs them both (for he taught all his household to know God, and to fear and obey him, Gen. 18:19). He judgeth them both to be in one state in regard of God’s covenant, though they were not; but that difference is made by God. Abraham doth not on his own head, and by his own will, put Ishmael out of the church, which was in his family; but God bids him put him out, and then he put him out, and not before; till such time he kept him in, and held him to be within the covenant, as well as Isaac was. Even so must we deal towards those that live in the church: secret judgment must be left to God; and (till God manifest the contrary) in the judgment of charity we must hold them all elect. This is the practice of St. Paul in all his epistles: writing to the Corinthians (1 Cor. 1:2), he calls them all sanctified; and to the Galatians (Gal. 1:2), he calls them all elect; speaking so in the judgment of charity, although he knew that among them there were many profane and wicked men, and though he reprove many great errors and heinous sins amongst them.