A rational person should be a strong atheist, that is, should reach the conclusion that no gods exist, and that no gods can exist, because:
1) one understand the materialist explanation of the origin of the universe, this eliminates the requirement for a god to create it.
2) one understands that postulating a god as an explanation for the universe just postpones the problem, there is still a requirement ot explain the origin of the god. If god does not require an explanation, why does the universe?
3) the idea of a god is incoherent. If you claim that your definition of "god" is coherent, please give it, I think I can show that it is not, but admit I am an amateur at this.
4) one understand the history of the creation of gods, and the facts of cultural transmission, which explain the popularity of the explanation without contributing anything to its believability.
5) one understands something of human psychology, at a minimum Asimov's "Six Security Beliefs" which explain the psychic comfort that the explanation provides, again explaining its poularity, despite its lack of truth.
6) one understands the psychology that allows the misinterpretation of various normal internal psychological events, such as dreams and feelings, memories of childhood dependency and the overwhelming feeling of love from a parent, the desire to submit to authority (such as a parent) which is always right and loving.
7) abnormal psychological events such as seizures, hallucinations, depersonalization, auditory hallucinations, whether experienced by the person or by an acquaintance are used to substantiate the god explanation, if one understands their actual source, the god explanation is unnecessary.
8) the competing varieties of god explanations indicate that they are cultural artifacts, responses to the kind of internal events such as those mentioned above, rather than descriptions of something real, which would tend to be all the same if there were and actual referent.
9) knowing how poorly humanity interprets probability, and how susceptible it is to wishful thinking, one dismisses the evidence of prayer and meditation.
10) the histories of the god explanations show that they have never delivered on what they promise. While promising peace, they have delivered conflict, while promising love they have encouraged hatred, while promising protection and supernatural help, they have delivered what looks suspiciously like random events.
Well, I am running out of time. Those are the first ten reasons why a rational person should conclude that there is no god, and why there cannot be a god. The one that need the most expansion is the one about incoherence, which has mainly to do with the explanation of how a god transcending the universe can act within it, or how a compassionate god can cause such misery, or a loving god such hate.
I hope you don't mind, these have gone out to the lists errancy and sechum-l.
-- To know that the Bible is the literature of a barbarous people, to know that it is uninspired, to be certain that the supernatural does not and cannot exist--all this is but the beginning of wisdom.--Robert G. Ingersoll / Greg Erwin, VP, Humanist Association of Canada