Sudden senseless breakup

We're both in the army right now, really close to getting released. We had an amazing relationship for over 2 years in which we loved eachother consistently and without regret but something changed in a very sudden and dramatic fashion last week.
She had to go north in order to guard a base for a week and right before she did everything was great as usual, and i'm quite a perceptive and empathetic person, i'd notice immedietly if anything was wrong.
She came back and already everything was different. She'd talk to me way less and didn't say anything affectionate at all.
After a few days we met up and she told me that she had decided to become religious, even though except for a few comments about it, this was the first time i've seen her taking it that seriously, and all that in a little less than two weeks!!
She told me that some guy who just became extremely religious talked to her alot during this time and that he made her realize that this is something she must do and that I can't be present during that process.

Anyway, it's been a few days, everything has suddenly gone to shit, the closest person I ever had left me over what seems to be an insanely irresponsible impulse, and I can barely function.

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then you obviously dont and did not ever bother to find out who she really is.
if she wants to be apart from you in order to fulfill her religious dreams (i think thats bullshit personally but here you go) then all you can do is to accept that.
its shit.
all you can do is to whish her luck or to try and find out her reasoning behind it.

and its not all of the sudden like you say
it has been there before you just didnt take it seriously

Honestly, I think it sounds really like a bullshit but who knows, maybe she really is serious about it.
Anyway, I feel like if she really loves you, she would include you in the plans. A sudden break up like this is kinda irresponsible since you guys had been together for 2 years.

If you accept the current situation, then you should just let her go and try not to be in the sorrow. If you don't, go talk to her again but don't be so clingy.

Even if the religious is true which I think it's a bullcrap story, it's only an insanely impulsive to you. She decided that's what she wants and you unfortunately need to live with it. What religion is it? Why in the world can't someone be together? Is she going on a pilgrimage? Is she thinking of becoming a nun?