gerrymander61 said:
Religion is all about faith. Chell had faith in that the cake existed and wasn't a lie, and in the end was rewarded for her faith in that she left Aperture Science for a better place. If Portal is meant to be compared to religion, then you have it the wrong way around. "The cake is a lie" isn't meant to ridicule the idea of religion, but rather strengthen the ideas of perseverance and faith, two very important parts of religion and human spirituality.
Let's assume that Chell = humanity, GlaDos = God and Cake = Heaven.
We assume she initially had faith in the promises of cake by GlaDos. She went through all the tests she was prescribed. Then she turned AGAINT GlaDos when it is revealed GlaDos is insane trying to kill her in the same way it is implied she killed all of Aperture Science. Chell even goes so far as to KILL GlaDos (possibly giving Neitszche something to think about) and then gets a 'reward' far DIFFERENT from the one she was promised. A reward she couldn't have gotten without metaphorically killing God - being set free from the lab and released into the 'real world'.
Remember, at the end when we finally see the cake, it is implied that it is a trap. (The pincher comes down) And even then, the cake recipe (you do hear it, but it can be easily missed, I recommend you look for it on YouTube) would produce something utterly inedible and probably toxic.
"The Cake Is A Lie" means just that; the promise of a reward is in fact nothing more than a false motivator by an ultimately insane overlord that must be overthrown and destroyed in order for true liberation.
But hey, that's just my theory.