Is there a slightly ironic idiom for "one's purpose in life"?

In one of my two native tongues there's an expression which, if literally translated, would sound somewhat like: "to make it one's flag"...

It may be used in a positive sense, connoting that someone has really devoted himself/herself to exercising his/her ideals in practice - and chances are that this is the prevalent usage - but it can also be amenable to a pinch of irony when such devotion is shown towards things that are commonly admitted as repercussions of vanity, or perhaps even sheer foolishness.

Here's a few examples where it might be used in the latter sense:

Getting together with your pals for a poker evening every once in a while is one thing. Making gamble your flag is quite another.

I can understand he finds it difficult to cut smoking; but that doesn't mean I approve of him strolling around, making “anti-hygienism” - or whatever you wanna call it - his flag!

I wonder if there is some English half-slangy (non-offensive) idiom that might do the same trick?

(Hope this one's clear enough).

Solution 1:

May not meet all of your requirements, but how about modus vivendi? For me, it conjures the sense of your "make it one's flag".

Dictionary.com:

modus vivendi: manner of living; way of life; lifestyle

Your examples:

Getting together with your pals for a poker evening every once in a while is one thing. Making gambling your modus vivendi is quite another.

I can understand he finds it difficult to cut smoking, but that doesn't mean I approve of him strolling around, making “anti-hygienism” his modus vivendi!