-- Oxford English Dictionary, 2nd edn
who is the subjective case; whom is the objective case.
But whom is rarely used nowadays. Except in the most formal of prose, it is usually acceptible to use who for either role. (The giveaway above is the archaic doth , which is unlikely to occur with a 'modern' use of objective who .) Care should be taken not to write something that is potentially ambiguous:
People who know the who/whom distinction will think this says "The man who loves God is happy", with an archaic or poetic choice of positioning the verb. People who don't know this means of discovering the subject of loves will use proximity instead, and will think the sentence means "The man who is loved by God is happy". Losing the distinction between who/whom means that we lose the freedom to write the words in any order we like.