You raise some very interesting points, fiendik.
First I don't completely agree with your following claim: "However, I should point out that the difference between those "known for Christianity" and those "known for philosophy" is much more basic than it sounds; one is beginning with the assumption that as a being that did not create itself, the philosopher is not in a position to determine truth on their own, and must look outside themselves for metaphysical information. The other begins with the assumption that outside sources are not to be definitely trusted, and therefore the only source of truth is the philosopher's own observation."
It is certainly right that philosphers are reluctant to trust outside sources but I think it is a stretch to say their only source of truth is their own observation. Of course, for anyone including the Christians, they to some extent base their view on their observation. You are the one who holds the view that you did not create yourself, and others hold other views. However neither you nor the philosopher (if he is a good philosopher) will stop at that point. The philosopher will search for truth through logic, reasoning, the evidence provided by science, emotions and so on. Good philosophy is based on the idea that what I provide as evidence is so strong that you cannot disagree or if you disagre and can prove me wrong, then you are right. So it is not a purely subjective "own observation" at least not in good philosophy (in my oppinion - the subjective aspect keeps forcing its way through
I think you correctly identify the challenges for moral philosophy in the following statement: "The problem comes when we try to independently determine what the law is. As a created being, I can say that my Creator had a purpose in mind for me, and that I must align myself to that. But without that, we don't really have any way of determining what law we should be following. Prior to Christianity, many people thought that eating each other was morally right. How can you say that they were wrong? If moral obligation is really an external force, than what does it actually affect? We know the force of gravity exists because of the objects which it influences. There is obviously no moral code that forces everyone to comply with it; Even if we were required by some mysterious force to "do good for goodness sake" then how would we be able to know what was really good?"
The first part about "independently" needs a little balancing as I have pointed to above, but basically you are right that it starts with the individual. As I have tried to point out at an earlier point I don't see that pointing to God as a creator with a purpose helps solve the problem. What is his purpose based on? If it is based on reason, fairness, a wish for happiness or some other basic principle or principles in combination then human reasoning would be able to grasp at least some of it though perhaps not to the perfect degree. This is by no means undermining the concept of God. As Leibniz said there are some basic truth that cannot be diffent under any circumstances. God would not be able to create a universe in which "2 + 2 = 5" would be true, simple because that statement is of such a form that it cannot be true under any circumstances. Contradictions cannot be true and thus God, according to Leibniz, would never be able to make them the true.
His points about ethics follow that same line of reasoning. We could not imagine a world in which torture of babies would be just - if the word "ethics" is to have any meaningfull content it cannot contain that. We could not image moral values being just about anything. Or, if we could, then we are basically just following a leader blindly with no idea of justice or truth at all. We would basically be nihilist people apart from the one single value that we would need to follow God, though we have no moral justification for doing so and no reasoning to point to as binding for any non-believer. As I have allready quoted we would worship God no matter what he did - even if he wanted to torture babies, make fun of the disabled or whatever.
Put in very simple terms; if justice does not exist without God, then it is very hard to see how it exists with God. If it is to be values that are justifiable and compelling they need to be within the grasp of reasoning. Or else, basically, we are in the situation in which many extremists find themselves. Religious extremists often claim that values exist solely due to the fact that God exists so any norm, law, philosophical idea, convention, human right or whatever can just be ignored. And then they unreflectingly kill.
You wrote: "There is obviously no moral code that forces everyone to comply with it; Even if we were required by some mysterious force to "do good for goodness sake" then how would we be able to know what was really good?"" Let's not forget that this goes for the moral code in Christianity as well. No one is forced to comply with that either (at least in this world). The evidence in this world for that set of moral principles is no stronger than the evidence for some other set of principles.
So basically I think you are completely right about the challenges for moral philosophy. It is very hard to base an ethical system. I just don't see how pointing to God makes a difference in that context. I think all people face exactly the same challenge - difficult as it is. I may write some more on that later but I think i have used my quota right now.
And that part would take very, very, very many words.