What must you believe in order to come to God?
To come to God, you must believe by faith that He exists—that He is real—and that He rewards those who diligently seek Him. Scripture says that without faith it is impossible to please Him, so an evil heart of unbelief is what prevents someone from entering into His rest.
Les explains from Hebrews 11:6 that faith is one of the two absolutes you cannot detour around or compromise. Without faith it is impossible to please God, and the one who comes to God must believe that He is, that He is for real, and that He is the rewarder of those who diligently seek Him. Earlier in Hebrews, Les warns that the evil heart that keeps people from God's rest is not chiefly covetousness or immorality but unbelief, and that warning is for us as well.
But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.