A
religion that allows its
believers to
modify,
change, or
adjust it to fit the
individual, or to
update things that are
obsolete.
A number of Eastern religions are more like this, but it especially applies to ones that are not organized, such as Wicca.
The opposite of many Western authoritarian religions, though if you think about it, they've got a little bit of this also, after all, the books most use as the ultimate authority are not clear and usually only interpretation yields any concrete beliefs, and those interpretations can change over time, kind of like saying that the truth is just what everyone believes is correct.