So Jesus said to the Jews who had believed him, “If you abide in my word, you are truly my disciples, and you will know the truth, and the truth will set you free.”
Putting this little promise in its context makes it even more powerful. Here is what is going on: Jesus is in conversation with Jewish leaders. They had a hard time tracking with what He has just told them about God the Father (John 8:27). So Jesus goes on to predict their future crucifixion of Him (John 8:28) and how that will change everything. And He makes what had to be to them an audacious claim of sinlessness by declaring that He always does the will of the Father (John 8:29). And the result is that many in the group started to be persuaded and to "believe" (John 8:30).
Jesus turns to address just that group in verse 30. He issues them a call to be His disciples, which always follows belief... Belief in the gospel = following Jesus. And in that call we see the power and the beauty of the Word of Christ. To be a follower of Jesus means we live in, by, through, and according to His Word. We abide in it. We dwell in it. We take up our home in Him. We live in what Jesus says. That is the mark of those "truly" His disciples and that is according to Jesus' own criteria in this passage. You can't say You love Jesus, but have low regard for the Bible. That isn't loving Jesus.
Jesus promises that His Word will show us the truth. Know Jesus by knowing His Word and You know His truth. Know His truth and truth will bring the ultimate freedom! A follower of Jesus is not bound by rules and dictates, but is set free in truth and real relationship with God!
The end of the conversation shows that though these Jews were intrigued by Jesus, they would not live in His Word, which was a clear rejection of Him. They trusted instead in their natural birth as Abraham's descendents and did not enter into a new birth in Jesus (John 8:33-37). In the end Jesus tells them that Abraham is not their father, but rather the Devil is, whose evil will they were doing (John 8:42-44). You can't be both a child of the Devil and a disciple of Jesus. And they showed their allegiances because they could not "bear" Jesus' words (John 8:43). And thus they could not know the truth that sets us free.