Like it or not, man has no choice but to do exactly that. When one person (e.g. Mother Theresa) says that God wants you to help the poor and another person (e.g. Osama bin Laden) says that God wants you to slay the infidel, you must fall back upon your own reason to decide which of these assertions (if either) is a worthwhile guide to behavior.
By what standards shall we make such "reasoned" judgements of value?