I was halfway joking with my list (especially 4), but I think that 3 and 5 are valid.
You are supposed to live mindfully, conscious of the cost you impose on the environment/Earth, and also to have fun and do as you like, within reasonable bounds.
Of course you can apply reductio ad absurdum and say, for example, that you can't bike because it contributes to carbon emissions and thus to global warming, which is bad for the Earth (that politician was so ridiculous). But I think "reasonable bounds" covers most of it.
The reason that I agree that Mustachianism is a belief system and quasi-religion is because I've converted people to it (I suppose that I object to blind zealotry as well). My sister and her fiancé have read the comprehensive Zero to Hero post (as well as other articles) and agree that they would like to spend more time with their children than the average two-worker household would. Her fiancé, without any input from me or anybody, already saves 50% of his income, so I suppose that I deliberately sow on fertile ground. One of my former colleagues and a good friend also read the Zero to Hero post and loved it; she is in the process of listening to every MMM post through SoundGecko. I mentioned MMM to her when we were discussing how her mother has made her own hygiene and household cleaning products for a very long time and provides food for her family through gardening.
I've also tried to convert two other people. One has agreed that early retirement is a worthy goal and something he's interested in pursuing when he's actually making money. He uses his bike or his own two legs for most things and minimizes his car use for financial and environmental reasons. The other person, my best friend, wildly out-Mustachians me to begin with, so I guess that I didn't really convert her so much as show her another facet of environmental consciousness and financial responsibility.
I don't think that Mustachianism replaces or displaces a religion (I'm pretty deeply religious), but I definitely agree that it shares traits with at least a belief system.