What does Hebrews 5:13 mean?

The context of this verse is spiritual immaturity. The writer of Hebrews has paused his explanation of Christ's High Priesthood to give a warning about those who take a lazy, slow-moving approach to their spiritual growth. In the prior verse, he pointed out that the Jewish Christians he writes to ought to be teachers of the faith by now. Instead, they are still in need of basic instruction. In this way, they are like babies who have never grown up: they are still dependent on milk and cannot handle solid foods.

Those who come to faith in Christ have to begin with basic ideas (1 Peter 2:2). This is a good thing, since it promotes growth. Over time, though, a believer should learn, and develop, and deepen their understanding (1 Corinthians 3:1). The reason these particular Christians have not grown is given in verse 11: they are "dull," from a Greek word meaning "slow or lazy." This criticism is similar to the one given in Hebrews 2:1–4, admonishing the readers to "pay much closer attention" to matters of faith.

Those who are bland and disinterested in their spiritual growth simply will not grow. Spiritually, they are like babies, who can only consume the simplest of things, and have no ability to process anything of real substance. The term "unskilled" here is from the Greek apeiros, which means someone who lacks experience.