Valid in this context means the statements the religion makes about the
world are true. At least I think that is what I mean. :-)

Then...

(1) The person is now in a religion R whose only precept, Y, is true,
making the religion R valid.

.... the definition of religion implied above is extremely weak. To wit:
"A religion is a set of true statements" is a poor definition of
religion. I suppose that's your point (and I agree with it). However,
R being invalid does not prevent R from being a religion, any more than
X being invalid does.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.