Abraham's
Compare (John 8:39).
The contrast, "I know that ye are Abraham's seed" -- "If ye were Abraham's children," is that between the natural and the spiritual posterity of Abraham. The Israelitish people and Ishmaelites are the former; all who are "of like precious faith with Abraham," whether Jews or Gentiles, are the latter; (Romans 9:6-8); (Galatians 3:6-14).
See "Abrahamic Covenant," ( See Scofield) - (Genesis 15:18).