According to Promise (Paperback) (ISBN-13: 9781943133109)

Isaac and Ishmael were both sons of Abraham, yet only one was the son of promise. For the people of God, living in covenant relationship with him requires understanding of what it means to be people of the promise. In this book Spurgeon explores the promises of God to his covenant people and the life that his people are to live as a result.

"O friend, if what you have within you is natural, and only natural, it will not save you The inward work must be supernatural; it must come of God, or it will miss the covenant blessing. A gracious life will be your own, even as Isaac was truly the child of Abraham; but still more it will be of God; for "Salvation is of the Lord." We must be born from above. Concerning all our religious feelings and actions, we must be able to say,

'Lord, thou hast wrought all our works in us.'"