Faith is a relationship; it is a lifeline between God and the regenerated man.  But without faith it is impossible to please Him; for he that cometh to God must believe that he is, and he is the rewarder of them that diligently seek Him, Heb 11:6 (KJV). Faith is the strength and vitality of Christianity. Faith at work is Christianity on the move.

The just shall live by faith simply means life can only be sustained as long as faith is at work. Outside of faith man merely exists. It is faith that turns existence into living.