Sentient beings (angels and humans) were created with awareness/consciousness - of themselves, of their surroundings, and of their sensations. They were created with the capacity to consider their options and to make choices, based upon expected future outcome - as opposed to always acting from instinct. They did not loose that normal genetic trait when Adam sinned.
Any normal human being has the capacity to believe, if he is given the information upon which to base belief. Hence the command to "go and teach". The only things that would hinder/prevent belief are 1. lack of information and 2. the desire to avoid compliance with the Law of God
Samie, I feel you take Christ words "without me you can do nothing" to EXTREME. Do you think human beings cannot physically walk, or see, or scratch their heads - "without" Christ? Of course they can.