What does John 10:8 mean?

Jesus is presenting the second of three metaphors for His ministry. These are related, but separate from each other. They are directed at the local religious leaders, who criticized Jesus for giving sight to a man born blind. In the first analogy, Jesus implied that those who reject Him do so because they are not part of "His" flock—they are following someone other than God.

In this example, Jesus builds on that prior idea. Sheep pens in that era were constructed with a narrow opening—and only one opening. This allowed the gatekeeper to easily control which animals came in or out. Anyone who tried to enter the pen by another path, or take animals out except through that door, was a thief by definition. To sleep or rest, the gatekeeper would lay across the opening, literally becoming the door. In the prior verse, Jesus claimed to fulfill that role in a spiritual sense, saying, "I am the door." This was the third time in the gospel of John that Jesus evokes the godly identity Moses heard from the burning bush in Exodus 3:14.

Here, Jesus continues to explain that, like the door—the gatekeeper—He is the one and only legitimate authority for the sheep. The religious leaders who have controlled Israel to that point are like those trying to sneak into the sheep pen, or calling to sheep which are not theirs. They are, spiritually, thieves and robbers.