What kind of evil heart does Hebrews 3:12 warn believers against?

Les explains that Hebrews 3:12 says, "Take heed, brethren, lest there be in any of you an evil heart of unbelief." He notes this warning is for us as well. The evil heart is not described as covetousness, immorality, or anything else one might think of, but specifically as unbelief. This connects to Hebrews 11:6, where without faith it is impossible to please Him, and he that comes to God must believe that He is and that He is a rewarder of them that diligently seek Him. Les calls this one of the absolutes that cannot be detoured around or compromised. In the earlier context, God was grieved with that generation, they always erred in their heart and did not know His ways, so He swore in His wrath that they would not enter His rest. They could have had it, but unbelief kept them out.