Most dictionaries include the following definition(s):

"n. 1. a. Belief in and reverence for a supernatural power or powers
regarded as creator and governor of the universe. b. A personal or
institutionalized system grounded in such belief and worship.
2. The life or condition of a person in a religious order. 
3. A set of beliefs, values, and practices based on the teachings of a
spiritual leader. 4. A cause, a principle, or an activity pursued with
zeal or conscientious devotion." -
http://dictionary.reference.com/search?q=religion

It is the 4th definition that causes me consternation: "4. A cause, a
principle, or an activity pursued with zeal or conscientious
devotion."

I may be an atheist but I do not "pursue" atheism with "zeal" or with
"conscientious devotion", I simply have no belief in deities/gods/the
supernatural. I follow no particular strand of atheism (I have to
figure this out! I wasn?t aware there were ?varieties? of atheists),
don't call myself a ?humanist?; a ?positive atheist?; nor consider
myself a "weak atheist". I simply do not care about the supernatural
as I have no belief in it. I neither belong to any organisation that
stands up for/promotes atheism nor seek that others "not believe".

I have been confronted that I do indeed follow a religion: atheism;
and it is the strict adherence to that definition that stumps me. That
4th definition is new to me and seems to be against what I thought the
word "religion" was defined as: the first 3 definitions. Or am I faced
with sophistry?
While I accept that there may be atheists who are quite vocal in
defending their rights ?not to believe?, are they followers of a
religion then?

Where/when did that 4th definition apply to the word "religion", or
should I be asking: Since when has the word "religion" come to mainly
mean belief system in a supernatural being(s) which led me to be
puzzled that religion can apply to atheism and thus I ?am following a
religion?? (i.e. Is the 4th definition a modern interpretation, or an
original meaning that now seems at odds with the main-stream meaning
of ?religion??)

Is the 4th definition correct on its own but when defined with atheism, not apply?
Help!