What did Jesus mean that if they were Abraham's children they would do the works of Abraham?
Jesus meant that merely being Abraham's physical descendants was not enough. True children of Abraham imitate his faith and deeds. Because they sought to kill Him rather than embrace the truth He heard from God, they proved they were not doing Abraham's works, since Abraham did not do such things.
Les explains that the people Jesus confronted were relying on their genealogy—claiming, “We are the children of Abraham, Isaac, and Jacob”—and therefore thought they had no sin problem. Jesus acknowledged they were Abraham’s seed physically, but He exposed their hearts: because His word had no place in them, they sought to kill Him. In verse 38 He said He spoke what He had seen with His Father, while they did what they had seen with their father, showing they were not of the same Father. When they insisted, “Abraham is our Father,” Jesus answered that if they were Abraham’s children they would do the works of Abraham—they would be people of faith. Instead they sought to kill the One who told them the truth from God. Les points out that this work—rejecting and seeking to kill the truth—was not what Abraham did; as Abraham’s sons who thought they were righteous, they should have embraced Jesus and recognized who He was, but they were blind.
They answered and said unto him, Abraham is our father. Jesus saith unto them, If ye were Abraham’s children, ye would do the works of Abraham.