Faith is the ability to believe that something is true, not only in the absence of supporting evidence but in the face of an overwhelming body of evidence that it is false. Religious faith is held by its adherents to be the essence of righteousness and is completely unassailable by reason. In any other context, an unshakable belief in a fantasy is a defining symptom of insanity. 
Rationalism is basically about thinking rationally (you guessed that didn't you). Rationalism is also about following reason and the Scientific method. Rationalists avoid Faith based belief systems because faith is not based on empirical, scientific evidence.