But it's not talking about salvation. It's talking about Israel. It's about God fulfilling His promises to Abraham. Note that Paul says the promises are the fathers. What are the promises he speaks of? They can be found in Genesis. God promised that He would make Abraham the father of a great nations, the father of many nations, that all nations would be blessed through him, and all the land that he could see. God also promised the land to Isaac and Jacob. These are the promises that are the fathers. It's the fulfillment of these promises through the nation of Israel that he is writing about. How and through which individuals God fulfilled these promises was based on His election of people. He elected Issac and not Ishmael. He elect Jacob and not Esau. He chose who He wanted to choose. They were chosen before birth so they had no input into their election. There's nothing in this passage that talks about salvation. Notice how he starts.
I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost,
2 That I have great heaviness and continual sorrow in my heart.
3 For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh:
4 Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises;
5 Whose are the fathers, and of whom as concerning the flesh Christ came, who is over all, God blessed for ever. Amen.
6 Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:
7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed. (Rom. 9:1-8 KJV)
He says it's not as though the word of God has been of no effect. He then goes on to explain what he means. The physical seed of Abraham are not the seed. Remember, he's addressing the Jewish portion of the church at Rome. He's telling them that it's not the physical seed of Abraham that are the children of the God, but rather the children of the promise. The seed is coming through the line of Isaac, not Ishmael, through Jacob, not Esau. God will choose who He will choose. Remember, Israel is the elect people of God.