Well, there’s a problem here, and so we come to the problem of degeneration or the danger of degeneration--the third warning passage. The first one is the danger of drifting. The second is the danger of disobedience; and now we have the danger of degeneration, the problem with dullness. It is obvious from verse eleven that the readers were characterized by stagnation. The word "sluggish" in 6:12, also meaning lazy, is the same as "dull" in 5:11. The perfect tense "you have become" used verse 11 implies this had become a fixed condition in their spiritual state. To the writer of Hebrews this dullness may be a precursor to degeneration; that is apostacy.

The doctrine of Melchizedek and his relationship to Christ was both full and difficult (5:11). It was especially hard to explain because the majority of the readers were immature (5:12).

Verses 13-14 point to one evidence or mark of maturity. This is the ability to discern (cf. 1 Cor. 2:15), and it is one of the evidences of maturity that you see almost consistently throughout the New Testament. The discernment does not come from "book learning" but from the practice of training spiritual senses (cf. 5:13-14). It is consistent practice and training.