Fullmetal Alchemist holds a very powerful question at its core: What is equivalent to the value to a human life? How valuable is a soul?
In the world of Fullmetal Alchemist, alchemy can be used to transmute any material to one of equal property and value. The human body has been broken down to all of its base elements to the exact number, yet nobody has ever successfully transmuted a human being, due to "something missing" - a soul, presumably. One of the characters in the graphic novel - Father Cornello - refers to the attempt at human transmutation as "the worst possible sin" - as it is akin to playing God.
When the Elric brothers attempt to perform human transmutation to bring their mother back to life, Edward loses his leg, while Alphonse loses his entire body, and that cost is only enough to bring something monstrous and unhuman back - it still wasn't enough to regain their mother.
Refusing to lose his brother as well, Edward sacrifices his arm in order to bind Alphonse's soul to a nearby suit of armor. This proves that souls DO exist, yet there appears to be no way to transmute them (for what could be equal in value to a soul?) so they can only be bound to objects
The Elric brothers attempted to transcend their humanity by bringing someone back from the dead, and in turn paid an unimaginable price. The story implies that it is not a human being's place to put a value on the soul, nor their fellow human.
There is a plethora of religious iconography in Fullmetal Alchemist; We see Father Cornello as a false prophet, Lust and Gluttony as punishing and monstrous embodiments, and the implications of "playing god" - as well as the repercussions of the act
I'm sure reading more volumes of the story would reveal more, but there's no need to spoil anything - I would risk flying too close to the sun