Address the point raised which was not about Isaac
Once one believes, he becomes a brother in Christ.
And faith is the grounds upon which one receives fruit
Romans 4:16–17 (LEB) — 16 Because of this, it is by faith, in order that it may be according to grace, so that the promise may be secure to all the descendants, not only to those of the law, but also to those of the faith of Abraham, who is the father of us all 17 (just as it is written, “I have made you the father of many nations”) before God, in whom he believed, the one who makes the dead alive and who calls the things that are not as though they are,
Colossians 1:2–6 (KJV 1900) — 2 To the saints and faithful brethren in Christ which are at Colosse: Grace be unto you, and peace, from God our Father and the Lord Jesus Christ. 3 We give thanks to God and the Father of our Lord Jesus Christ, praying always for you, 4 Since we heard of your faith in Christ Jesus, and of the love which ye have to all the saints, 5 For the hope which is laid up for you in heaven, whereof ye heard before in the word of the truth of the gospel; 6 Which is come unto you, as it is in all the world; and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth:
It is a believer who is the Spiritual child of Abraham
Galatians 3:7 (LEB) — 7 then understand that the ones who have faith, these are sons of Abraham.