In all theistic systems--even non-theological mystical systems--there is the assumption of a supernatural realm that transcends normal human perceptions, giving meaning and validity to a person's life in exchange for some sort of obedience or submission to a prescribed way of living (believing, behaving, or worshipping) in accordance with the respective theological or mystical system.
In a non-theistic and non-mystical system, there is no belief in any spiritual realm outside of a person or transcending him or her. The realm of love, reason and justice exists as a reality only because, and inasmuch as, a person has been able to develop these powers in himself or herself throughout the process of his or her self-awareness, self-improvement, and eventual self-fulfillment.
In this non-theistic and non-mystical view there is no meaning to life, except the meaning that the person himself or herself gives to it. Such a person realizes that he or she is utterly alone except inasmuch as he or she cooperates or associates with another.