Abraham is our father - We are descended from Abraham. Of this the Jews boasted much, as being descended from such an illustrious man. See the notes at Matthew 3:9. As Jesus did not expressly say who he meant John 8:38 when he said they did the works of their father, they obstinately persisted in pretending not to understand him, as if they had said, “We acknowledge no other father but Abraham, and to charge us with being the offspring of another is slander and calumny.”
If ye were Abraham’s children - The words “sons” and “children” are often used to denote those who imitate another or who have his spirit. See the notes at Matthew 1:1. Here it means, “if you were worthy to be called the children of Abraham, or if you had his spirit.”