The answer is Abraham.
Jacob, also known as Israel, is a significant patriarch in the Bible, but he is not specifically mentioned by Paul in Galatians as the spiritual ancestor for those who have faith in Christ.
Joseph, the son of Jacob, is known for his story in Egypt and his role in saving his family during a famine. However, he is not identified by Paul in Galatians as the spiritual ancestor for believers in Christ.
Isaac is another patriarch and the son of Abraham, but Paul does not refer to him as the spiritual ancestor in the context of faith in Christ in Galatians.
In Galatians, Paul emphasizes that Abraham is the spiritual ancestor of those who have faith in Christ. He argues that just as Abraham was justified by faith, so are all who believe in Christ, making Abraham the father of all who have faith. This is based on Galatians 3:7, which states, "Understand, then, that those who have faith are children of Abraham."