This episode was billed as giving us the backstory of the age-less Richard Alpert, but for good measure they also threw in a few big revelations about the "Lost" mythology and the metaphysical laws of this very unusual land.
I think by now we've pretty much settled the good vs. evil debate. Jacob is the good that transcends religious beliefs and the baser instincts of humanity. The Man in Black is evil, the chaos of the dark side driven by fear an our natural kill-or-be-killed, survivalist impulses — the kind of thing that organized religion is designed to help keep in check — through fear, guilt, the carrot of heaven and the stick of hell. Whereas Jacob is something like faith in its purist form — no written tracts, no robes, no iconography or sacred symbols — just pure belief. Hence the seg title "Ab Aeterno," which is Latin for "From the beginning of time" (or so says my quick Google search.)
It takes a very special kind of soul to muster such conviction, such unadulterated feeling about man and the world and the spirituality that exists in every blade of grass, or carafe of wine. Religion is something that is used to control poor Ricardo — from the beginning when the priest wouldn't absolve him, right through the moment when he digs up his wife's cross necklace nearly 150 years after he buried it, because she finally absolved him of the guilt he felt for not being able to cure her sickness all those years ago.
MIB uses the terror of being consigned to hell to motivate Richard (aka Ricardo) to act on his behalf. MIB is so convincing that so many years later, as Richard has a crisis of faith he returns to MIB's lie and takes it to be the real truth because he's so disillusioned with Jacob. Jacob, on the other hand, in his first meeting with Richard is straight up with him about not being able to bring back his wife (as MIB promises) or even absolve him of the accidental killing of the greedy doctor. But Jacob can make Richard live forever with just a touch of the shoulder, which is pretty awe-inspiring on its own. There was a lot of that in this fast-moving seg written by Melinda Hsu Taylor and Greggory Nations and directed by Tucker Gates.