What does John 8:34 mean?

Here Jesus uses the phrase translated as "truly, truly," or "very truly." This is from a doubled use of the Aramaic word amēn. Used at the end of a statement, as many cultures do in prayer even today, it suggests a hope that the words will be fulfilled, or that they are true. Used at the beginning of a statement, it is a claim to absolute, original, first-hand knowledge.

It's important to realize what Christ is saying here, and what He is not saying. Earlier, Jesus claimed to be the one and only source of spiritual truth, and that those who accepted Him would be set free from the enslavement of sin (John 7:37–38; John 8:12). Without question, Jesus is pointing out that sin is a mark of following darkness, instead of His light (1 John 1:5-10). Sin, by definition, means choosing earthly, worldly things over heavenly things.

What Jesus is not saying is that all sin, at all times, should be interpreted to mean that the sinner has no relationship to Christ. The Greek of this phrase makes this nuance much easier to understand than any English translation. The exact phrasing used is pas ho poiōn ho hamartia doulos ho hamartia. Literally, this means "everyone who keeps practicing sin is a slave of sin." In other words, Jesus is now speaking of a habitual, persistent sin. Those who are free in Christ may stumble into darkness, but they do not perpetually "walk" in it (John 8:12).