Now we, brethren, as Isaac was, are children of promise. – Galatians 4:28

Ask the legalist, “You do good works; you have repented, you say; you are keeping the law, and you have no need to repent. Now, where did you get your strength from?” Perhaps he says, “Grace;” but if you ask him what he means, he says that he used it; he had grace, but he used it. Then the difference is, you used your grace, and others did not. Yes. Well, then, it is your own doing. You may call it grace, or you may call it mustard; it was no grace after all, for it was your using, you say, that made the difference. But ask poor Isaac how he has kept the law, and what does he say? Very badly, indeed. Are you a sinner, Isaac? “Oh! yes, an exceedingly great one; I have rebelled against my father times without number; I have often gone astray from him.” Then you do not think yourself quite as good as Ishmael, do you? “No.” But yet there is a difference between you and him after all. What has made the difference? “Why, grace has made me to differ.” Why is not Ishmael an Isaac? Could Ishmael have been an Isaac? “No,” says Isaac, “it was God who made me to differ, from the first to the last; He made me a child of promise before I was born, and He must keep me so.”

“Grace all the work shall crown
Through everlasting days;
It lays in heaven the topmost stone,
And well deserves the praise.”

~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0069.cfm