I think I've more or less cracked the requirements for classifying someone as religious. I'm not talking about how religious someone is - but just whether he is considered religious or not: By this I mean that most of us have a binary system of classifying people: They are either religious or they are not. However we accept a very wide range of observance from people classified as religious. And so what I am looking for are those few Mitzvot that are used by us religious folk to determine how to classify others on that binary system of ours. After much thought, I've got it down to just 3 Mitzvot out of 613:
The Must Keep Mitzvot:
If you are a boy:
1. Kosher food - someone who eats food considered by everyone as not kosher (Meat + Milk) is not religious.
2. Shabbat - Not observing Shabbos would definitely put you outside the religious community.
If you are a girl, the other two plus:
3. Tznius - This is the one I'm not sure about, but I think must enter the list for girls. I can't define what level of Tznius is needed to be religious - its as fluid a concept as you can imagine. However I am certain that people use Tznius as a foremost sign of recognition of the religious community. Those non Tznius girls walking down the street, can claim all they want that they keep Shabbos...mentally we don't classify them as religious.
Every other Mitzvah - I.E how often you pray, whether you put on Tefillin, whether you study Torah on a regular basis, whether you scare away a mother pigeon before stealing her eggs, are not used by us to make the binary decision of whether I am talking to someone religious or not - but just what level of religious the person is. We all know religious guys who don't study Torah or go to Shul. We don't consider them not religious - just less observant. However any guy who we see eating a shrimp - he's automatically considered as non religious. Same with Shabbos - break it on purpose and everyone around you automatically lists you as off the derech. These are the defining minimal levels of the homo-religious.