Why would Christianity be ''the truth"!!! Why wouldn't it be Islam, Buddhism, Taoism, or even Egyptian and Greek mythology.
Well
something is true, and many things are false. If Christianity is true then the base principles for Islaam are false, although the teachings might have some truth to them.
On a serious note Id like to say that though I am not one, I do believe Catholics are the only real Christians because they adore Mary as mother of God, bringing a balance of masculine and feminine, and they tend to believe in the actual teachings of Jesus ... You know, like take care of the poor and the sick and the dying.
From what I've seen Catholics like to do rituals not explicitly (or implicitly) prescribed by the Bible and have strange beliefs about exclusivity in the church through the Papacy due to very weak interpretations of one or two verses which really don't say much. Most, although not all, Christians that I see taking care of the poor are non-Catholic (and being from a missionary family, I've seen
a lot) although admittedly this could easily be a bias (I am tentatively a Protestant myself). That's not to say that Catholics aren't Christians, but I don't see how they're the only "real" Christians.
There can only be one truth. If one religion says that God is a pig and the other says that God is a monkey, they can't both be right. I guess she could be some sort of pig-monkey thing, but then there is still an objective truth, and both of them were partially right. Also, with such polarized religions it would be very difficult for them to both be correct, even in a partial sense. For many of their claims, either one is right or the other.
There is definitely one objective truth. I guess, like with your math analogy, there could be multiple ways of getting there/different understandings of the same thing, but there is definitely only one answer, and you can definitely be
wrong. If one person says the answer is "there is no value for x" and the other says "x equals five", they cannot both be correct. If one person says x is 3 and the other says x is 5, there is a very small chance that they are both correct, although neither would have the complete answer ( [x-5]*[x-3]=0 would yield x=3,5 as its answer). With truth claims being so different, it is highly unlikely that they are both right, especially when considering
opposing views (God is real vs. God is not real).
Also, while "all religions are essentially paths to the same thing" is often said with good intentions it is not only ignorant, but can actually be insulting to people with solid beliefs. You are essentially saying that one person's beliefs are the same as another's, and this can offend people. When you look into it, beliefs of different cultures are vastly different.
(highlighted because I feel that it is a point not frequently made and easily missed)