That depends I suppose what you mean with "rational", "reason",
"believing in", "God" an probably also "for" and "are " :o)
People make lots of experiences. Some of them come through
the "public senses", vision, smell etc. Our desire to make sense of
these and put them into a coherent whole gives us science and
its vocabulary. While it is theoretically possible that my senses deceive
me (all the time), I consider it rational to take as a working assumption
that they don't, and see where that takes me. Furthermore, even
though the "sense experience of reading an instrument" is initially a
personal, subjective experience, language allows us to communicate
about it an drake it objective (everybody who looks at the instrument and
has the right training describes what s/he sees similarly). When the
scientific vocabulary does its job, e.g. if the term "atom" helps me to systematise
quite a lot of sense data I have, it becomes rational to say "atoms exist".
Then there are other types of experience. Aesthetic experiences are a
paradigmatic example. They are less public and more personal, but
not less real. And even though they are not as directly linked to the
"public" senses, people very often find ways to describe them in ways
that allow them to communicate with people who have made a
similar experience. Just as with the experiences with external reality
described above, we have an inborn tendency to "make sense" of these
experiences, verbalise them, systematise them, and structure them.
That gives us aesthetic theory, literary criticism, but
also the strange vocabulary a sommelier might use to describe the
exact sense sensation he has when tasting a specific vintage. Furthermore,
people are able, despite the irreducibly personal nature of the experience
to improve the way in which they verbalise it (again, take the example
of a sommelier or a whisky aficionado, who will have a richer vocabulary
to describe his experience than a newcomer)
I'd argue that religious experiences are more similar
(not identical) to aesthetic experiences than
to the type of external sense experience that are at the basis of science. Terms like
"God", "transcendental" etc are just words that help us "to make sense of" these
experiences, structure and systematise them - and as with aesthetic experiences,
for some reason it seems that people who made this sort of experience can find
a vocabulary that allows them to a degree communicate about them with others
who made the same experience.
Now, there are lots of ways this can turn irrational.
The most frequent one is to mistake the sign for the thing, and treat the
attempt to verbalise an internal experience as if it was an external one
(a confusion of the senses, similar to calling a smell "green") I have a very
specific experience when listening to Aida for the first time - but that
does not tell me anything about "external" things like Egypt and the Pharaohs
Religious experiences, just like aesthetic experiences, are not primarily
referential, they are not "about" something.
Secondly, while it seems as if people who made a similar experience will
find ways to communicate about it, that does not mean that these verbalisations
are even intelligible for someone who did not make that experience. Proselyting
is therefore irrational, just as it is to convince someone that "atonal
music is really moving" when for them, it isn't. (and nothing wrong with this, the
only emotion atonal music triggers in me is a desire to kill the musician,
and a slow, painful peasant's death for that)
But it would be equally irrational for me to claim that I did not have that
experiences that I had. It would be equally irrational to ignore that the way
I verbalise them makes them intelligible for people with similar experience.
A term like "God" is legitimate if it helps to structure, systematics etc these
experiences in a intra-subjecitve way, if it "does its job". However, since the
input, the data that it tries to categorise etc is not the data we try to systematise
etc in science (that is, not data coming from the public senses), conflict is impossible.
Again as analogy, no amount of science will convince me that "Aida did not move
me after all" - I know it did - but equally, nothing I can say about this experience,
e.g. that it was like "being trampled underfoot by a herd of unicorns" commits me
to argue that unicorns exist the same way atoms do, or any other statement that
contradicts, even potentially, our best scientific theories