Our "3" in the context above, is our data. Our data, is our knowledge (right or wrong), personal experiences, etc etc.

The "2" is our intellectual tools that we have to process these data.

We all have our 3's and 2's and subjectively they always make sense - otherwise we wouldn't be using them as data, or as tools to process our data. Objectively, they will not always make sense, because others will have their own 3's and 2's (different values).

If A+B = C, then obviously the A of the christian and the A of the muslim will be different in value, because for each one, their data set about what god is, has different properties.

Obviously I cannot "blame" the muslim for being a muslim or the christian for being a christian. If you are raised in a certain way, given certain data about "god", develop or acquire certain intellectual tools, you'll end up with a certain belief system. Naturally, the disagreements and conflicts arising from each belief system are predictable, given how each one grew up. And, as you say, people will think that they are right, based on their own equations on their different data/numbers.