Jesus has just stated that if they were His disciples, they would be spiritually free. They respond by saying they are Abraham’s seed. Therefore, they were never in bondage. Jesus is speaking of spiritual bondage. Jesus says that if an individual is living in sin, they are in bondage. Note the following verse:
Romans 6:16 (NASB)
(16) “Do you not know that when you present yourselves to someone as slaves for obedience, you are slaves of the one whom you obey, either of sin resulting in death, or of obedience resulting in righteousness?”
An individual could be a slave, but it is “who” they give themselves to reveals the end result. Obedience to unrighteousness leads to slavery. Obedience to righteousness leads to freedom.