We all need to guard against unbelief as we would against an enemy. Paul is not speaking about a heart in which unbelief is merely present, but a heart that is controlled by unbelief, the kind of heart that will drag a person down even as Peter was dragged down into Galilee's water when he took his eyes off of Jesus. The peril of unbelief is that it breaks the trust on which our relationship with God is based. Unbelief leads to falling away, which is the opposite of drawing near. "Drawing near" is a major theme of Hebrews.
Falling away is the supreme disaster of life, the ultimate defeat, because it completely thwarts God's purpose for creation. It is essential we remember that when a person falls away, he is not merely falling away from a doctrine or even a set of doctrines, but from a living, dynamic Personality.
Faith needs to be cultivated. It grows by reading and studying God's Word, and by meditating on it. It grows in an atmosphere of trial or experience because it is exercised through use. It also grows, as we find here in these three verses, in an atmosphere of exhortation from others who are fellowshipping with us. Exhortation is a preventative of falling away, which is a major reason why fellowship is so necessary. Without it, a person may hold his own, and perhaps his faith will not slip very much, but one who is not fellowshipping with others of like mind will rarely ever grow.