Before trying to answer it, we have to make certain assumptions concerning the intent of the question "How can a thinking, rational adult be a monotheist?" The question can obviously be read in at least two modes:
- (1) rhetorically - the actual intent is to say "it is not reasonable for reasonable people to be monotheists"
- (2) explanatorily - the actual intent is to give reasonable reasons for being a monotheist
Defining
First, let us define monotheism, by saying that it is the belief which maintains that:
There exists an all-powerful, all-encompassing Entity, which is the creator of, reason for, and the ultimate controlling agent of Everything.
This may not square perfectly with all the theologies that we usually term as monotheist, but it still seems to express their central idea reasonably well.
(1) Interpreting the question in rhetorical mode
In this installment, let us examine the first mode of reading the question (the second mode will perhaps be treated in some later writeup). So let us try to make an argument for the thesis that reasonable people can not possibly be monotheists.
The only argument we actually need is that monotheism seems to be devoid of informational content.
Because if
- the Entity has created Everything (and nothing but Everything) from Everything (and from nothing but Everything),
- and the Entity is in complete control of Everything (and of nothing but Everything),
then the Entity can not be separated from Everything.
In other words, then Entity is the same as Everything.
No information content, hence no consequences
This in turn leads to a permanently true statement, a logical tautology of the type "Everything is Everything" or "Entity is Entity".
It is of course perfectly correct, but also perfectly useless, because it doesn't give us any information whatsoever about Everything. Hence monotheism has no consequences of any kind -- certainly no moral, ethical or practical consequences -- because by saying Everything about Everything, it does not say anything about anything in particular.
A thinking, rational adult would certainly not build his world-view on a statement that was devoid of content. Hence such an adult can not be a monotheist.
(An individual belonging to an actual monotheistic denomination might start bickering about such an argument, protesting that the Entity is in fact separate from Everything. But in that case Everything could not be termed Everything, not if the parishioner maintains that the Entity controls Everything, except for the Entity itself. Then the Entity clearly doesn't control Everything, it just controls "Everything, but not Entity". And this would be a bit less than all-powerful, wouldn't it -- not even being able to control yourself? Any monotheistic argument that separates the all-powerful Entity from what the Entity controls leads to logical problems, each leading to the creation of a new monotheistic denomination, possibly topping it all off by a new religious war.)
(2) Interpreting the question in explanatory mode
The interpretation of the question in explanatory mode, i.e. the interpretation that will lead to a list of reasonable reasons for being both reasonable and a monotheist, may follow at some later date, if written by an interested party.
APPENDIX
The argument (concerning case (1), rhetorical interpretation
of the question), presented in set-theoretical notation
The argument can be stated in more precise terms with the help of set theory:
Before creation began, only the element Entity existed. Hence the universal set EVERYTHING was at that time the set {Entity}. When creation got underway, a set of new elements (which we may call SOMETHING) was added, i.e. the union of SOMETHING and EVERYTHING was formed. But because EVERYTHING is the universal set, this union becomes SOMETHING ∪ EVERYTHING = EVERYTHING. (The union of an arbitrary set A and the universal set U is always A ∪ U = U ).
Hence:
{Entity} = EVERYTHING = SOMETHING ∪ EVERYTHING = EVERYTHING
Consequently the original set (which contains only the element Entity, i.e. the set {Entity}) is indistinguishable from the universal set EVERYTHING in all circumstances, whether there is creation, destruction of what has been created, or whatever.
This holds true under the "God is dead"-hypothesis as well, i.e. even when the element Entity is destroyed.
Obviously, EVERYTHING = EVERYTHING, whether there is a God in EVERYTHING or not.
This in turn necessitates an examination of the resulting tautologies: {Entity} = {Entity} and EVERYTHING = EVERYTHING.
Tautologies like DOG = DOG or 13 = 13 are perfectly true statements. However, they don't give us any information about the properties of DOG or of the number 13. A statement like DOG = CAT is obviously false, but it contains at least false information, and is in this sense informative (or disinformative, to be precise).
If the monotheistic thesis leads to a tautology, then it is devoid of information. And hence the monotheistic thesis, while being perfectly true, is seen to have no consequences whatsoever, particularly no moral, ethical or practical consequences.
NOTE: I have a vague memory that some early medieval theologian already discovered this problem. However, I don't know how he (it must have been a he) went about trying to explain it away. Please give me a /msg if you happen to have any information on this matter. | 
