“John 8:37 I know that you are descendants of Abraham. He says, as it were: ‘You make much ado about the fact that you are descended from Abraham and are of his flesh and blood. But that will not silence Me or disprove My words.’ This is just as if I, Doctor Luther, were to say: ‘I know that you are bishops and priests, that you are baptized, that you are invested with an episcopal office, that you are in the church, that you come from Christ. But that is not enough; it does not make you the true church.’ ‘Well,’ they say, ‘should this not help us, that we are among the Christians, that we are in the Christian Church, that we hold a Christian office?’ No, that will not do it. They have attached far more importance and significance than we do to their claim that they are the Christian Church. With this they meant to imply that no matter what we stress or what we preach, the Christian Church will not err; otherwise God Himself would err.
The Jews speak in a similar way here: ‘We are Abraham’s seed, children of God, God’s people, and Abraham is not a slave of God. But just as Abraham is a friend of God, so we, too, are God’s children and His people.’ Now the papists declare: ‘Since Christ does not err—and He is God—we also are the Christian Church, and the pope does not err.’ We know this well, and you need not teach us this. You are baptized, you are descended from the right people, you are numbered among the Christians, you can recite the Ten Commandments, you can pray the Lord’s Prayer, you can hear a sermon. But to say that because of this you are a Christian—not yet! That is not enough to make you a Christian. That is like the Jews’ statement that they are of Abraham’s seed. Christ says to them: ‘You are not free for that reason. You must come to Me and be delivered through Me, or you will die and perish in Your sins.’ But why is it not enough to be of the seed of Abraham? Were Abraham’s children not God’s very own people? Yes. But Christ says: ‘You are slaves to sin and seek to kill Me, because My Word finds no place in you’” (Luther’s Works, v. 23, p. 415).