6 But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel, 7 and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.” 8 This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring. 9 For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.” (Romans 9:5-9 (ESV)
So now we come to the promises of God. God is bound to his word. What God promises he brings about. In this way he binds himself, and thus the binding does not impinge upon his freedom, because in his freedom he chose it.
This is the answer to the sort of juvenile objections about putting God in a box as many think Christianity does. It isn’t Christianity that puts God in a box, it is just Christianity that recognizes the box God has put himself into for our sake. God can do what he wants. We, on the other hand, are not free to object to God doing what he wants. We can only recognize what it is he is doing, acknowledge it and put our trust in it.
Thus, if God wants to save us through Baptism (1 Peter 3), then we as Christians let God save us in baptism and trust that baptism. If God wants to forgive our sins through the pronouncement of forgiveness by fellow sinners (John 20), then we allow them to forgive our sins and recognize that God is working through them. If God wants to give us his body and blood for the forgiveness of sins through bread and wine then we bow before his presence and eat and drink. We do not say, “Well God can do whatever he wants, so I’m going to commune with him in nature because he is there too.” That isn’t communing with God. That is rejecting God. That is trying to have God on your terms, rather than acknowledging the terms he has given. God has promised to work salvation through his church and the means of grace that he has given his church, his word, and his sacraments. Therefore, we know he is active in and through his church where these things are found.
So God works through his promises. It is through promise that he deals with man. And this is also true of what he has done in the Old Testament. He dealt with Abraham through a word of promise, and promised to save his descendants. So what of the Jews? Are they lost? Well, not by virtue of being a Jew. But by virtue of unbelief. Here Paul explains. Not all are children of Abraham who are children of Abraham according to the flesh. Some who are not children of Abraham according to the flesh are actually Children of Abraham by faith. Father Abraham had many sons. Those who reject the promise made to Abraham, by rejecting Christ who is the fulfilment of that promise, are then lost, not because God is a liar, but because they have refused to believe his promise. So it is faith that saves, but faith believes the promise, faith holds to God’s promise and does not expect God to work outside his promise. Or as I like to say, faith doesn’t merely believe in God, or believe in Jesus. Faith believes God. Faith believes Jesus.