Abraham had two sonsJohn 8:31-45 depends on the story about two people who aren't even mentioned in the text; two people that Jesus depended on his hearers knowing about. They are Ishmael and Isaac. Jesus' hearers receive him as the promised Messiah but when he calls them to take him seriously and they would find freedom their pride rose up and overwhelmed any sense of pleasure they found in him.
Rome might have taken over the land but that didn't make them slaves. Try telling French or Belgian people during Hitler's reign that they were German "slaves". They would admit they needed rescued from Rome but to suggest they were slaves was outrageous—they were the sons of Abraham and national pride blotted out all else.
Their slavery was real for all that! Israel's history of unfaithfulness to God and the covenant was written all over the OT and this was proof positive that sin was their master. Their physical connection with Abraham guaranteed nothing! Yes, but they were Abraham's children and that had to count for something.
Jesus, though he doesn't mention their names, reminds them that Abraham had two sons—Ishmael and Isaac; one was free and the other a slave though both were Abraham's sons. [See Galatians 4:21-31 and this]
The only point I wish to make is: without knowing the story about Ishmael and Isaac we miss the point Jesus is making in John 8:31-45.
It's important if we're to become better interpreters of scripture that we get to know the OT as well as the NT. Richard B. Hays makes that abundantly clear in his Echoes of Scripture in the Letters of Paul and his later The Conversion of the Imagination.