There is some pretty interesting discussion going on here. I did not get to read every single post so please forgive me if I repeat some things that may have already been said.
I think one of the difficulties we seem to be having here is understanding the true definitions of philosophy and religion. I would agree that philosophy does try to indicate how people should live, though perhaps in its purest form it should not. When it comes to religion the problem is that for most people, religion is not merely a chosen belief system; It is an expression of the relationship that they have with God. Therefore, it would be almost impossible for their religion to not affect their philosophy (even Aquinas...if u study the bible *closely* and compare his studies you can see that).
NONETHELESS
.....I do believe that Christians and people of most other belief systems (including atheism..the belief in no God..lol) tend to approach the idea of existentialism with a presupposed bias instead of being open to ...dare I say ...truth. Truth has now become one of the most feared words in the english language and so we have conveniently declared that it does not exist or that is relative...but thats a whole other thread!..lol.
Either way I do firmly agree with what Mr. Self Destruct said about the biases we have..etc. What I notice is that many people do not truly know or understand what they belive and that is why when their beliefs are questioned they flee in ignorance...their entire life philosophy hangs on a thread UNLESS they truly know what and why they believe it.
This gets tricky in a conservative view of "faith" but what Christians (and I will only talk about christianity in this sense...and um....and that INCLUDES Catholics Forgottenknight) believe in terms of faith is that God is literally dwelling inside of them...hard to argue with if you have never experienced it. Still, just because you have been brought up in a church or etc....it does not mean that you have this personal relationship, it is all determined by a choice you make and that choice is initially spurred by faith. This is where the faith in God comes (as there are different forms of faith) and this is primarily a problem and issue for the nonbeliever, not the believer.
STILL....if Christians believe that they have the truth (as do all religions...yes ALL...some may say they dont believe this but when you talk with them more and read their doctrines and such you see that thats not true at all) then they should think they can "prove" their religion. Perhaps this has not been done because people are too indoctrinated to truly question their *DOCTRINES* in search of truth and the sad part is that these people only then hinder God.
I guess ultimately my point is that Christians and all religions should become much more philosophical about their belief systems. I am a Christian myself who has an inherent need to question everything..lol. I am a definite skeptic and yet in becoming more philosophical about my beliefs I have found the statement to be true that this does indeed only strengthen my "faith".
I hope this makes SOME kind of sense