I will respect the person for his/her un-belief in MY religion and belief in whatever else, and wish the person the very best as long as he/she does no harm or hurt others.
As to why he/she not believe, I will only say this: One’s beliefs and un-beliefs are dictated by many factors, ranging from birth in a family, early indoctrination, spiritual inspiration, influence of authors and speakers, etc.
It is impossible to give a definite answer to this question. I cannot and do not wish to analyze or pass judgment on others about this.
Spurts of Thought 