Reason ought to be the METHOD by which we explain and justify a belief system. For example, we could assume that pleasure is the starting point and that humanity is destined to seek out pleasurable experiences to enjoy life. Scientifically speaking, this is reasonable. Human brains are wired to enjoy certain activities (such as sex) in order to procreate and so both the starting point as well as the end point could be cemented by means of logic. If we wish to extend this and ask WHY science has us feeling this way by procreating, only THEN can we move to an unknown domain. By extending a rational belief system, it becomes irrational because we are no longer sure why we exist on this Earth in the first place. And so reason is perfectly capable of existing as a starting point to a belief system so long as that system is limited to what we are already aware of. The question did not specifically mention what kind of "belief systems" we are talking about.
Reason, I belive is everything. Without reason understanding will be very vague, and having a vague understanding of whatever system it is that you belive in is very dangerous. Reasoning completely eliminate chance of errors made by your emotions. Some people let their emotions to do the thinking for them, but it should really not be that way, your emotions can be miss guided, is their is no real scientific evidence proving the accuracy of emotions. And yes some Bible believers throw out reasoning and rely on faith along, which I think is very stupid. Their thinking is totally base on their emotions, that is why when someone who can reason read the Bible
As you grow, you take in information and begin to think; attempting to understand your surroundings. A belief system is simply a system to explain something. It doesn't always have to be supernatural or anything ridiculous. It could be your morals. So, as you grow older and realize that helping someone out could benefit you in the future, you create a belief system based off of your knowledge and reasoning that it would benefit you more to help someone, then ignore their problems. This is a legitimate belief system created from reason and if we are discussing supernatural belief systems, such as ghosts, we are still discussing reason... Let me explain. When you can't understand something; when it is "unexplainable," you search for the most applicable answer and often times this is supernatural cases. These belief systems are a filler for lack of knowledge. We are still using reasoning for what we couldn't reason before. Basically, reason and belief coincide. You notice patterns and make preset judgments on the knowledge you have found. These are your beliefs.
Reason is good when you already know what you ultimately want. But it can't be used to decide what to want except in relation to an already existing want.
Why do we want X? Because Y. Why do we want Y? Because Z. Why do we...."
That's why reason can't be the starting point. You could make being alive the starting point. You could make love the starting point. You could make promoting the progress of mankind your starting point (though then there's the question of what that means, same for love). You could make promoting the color yellow your starting point. What ever. But reason can't get you the starting point.
You may even have a starting point that's hard to put into words and includes many considerations and still have a good guide to action.
We should base our beliefs off of a fixed point of reference. A fixed point of reference has to requirements: 1. It must be fixed. It can't change. 2. It must be outside of you. Reason does not uphold these two requirements. A person's reason can change as they think more deeply on a subject, and a person's reason is obviously a part of them. Reason should not be thrown out the window, though. It has a place, it helps you choose your fixed point of reference and helps you explain it to others. However, it cannot be the fixed point of reference itself.
A belief-system is something you feel. It should comfort you, guide you, inspire you, etc. If that is a reason for believing, then perhaps that's enough. But, often times, reason & logic involve tangible proof. Faith isn't a lab experiment. It isn't Science. If you are looking for reason & logic in faith, you will fail miserably. Believing in any faith is a journey. Until you actually experience it, you cannot measure its significance on you or your life.
In that sense, a belief system is something that simply can not have its roots on reasoning. For, if there is reason, then there is objective knowledge. The concept of belief is more related to something that you can not or may not have fully comprehension of. So, reason alone would not be a good starting point for a belief system.
Reasoning without evidence (Platonism) produces delusion, not objective truths nor facts. Reason is like a blind man predicting the movements of the bodies in the cosmos. It models a world in the way the mind wants to believe it to be, not how it actually is. Science is studying the way everything actually is and comparing this to multiple observations of the same objects and systems for clarification/verification. Science is the only truly objectively relevant starting point for a belief system.
If a god demonstrated it's existence in a way that is scientifically verifiable and confirmed as god, I too would be a Theist.
But, such an occurrence appears highly unlikely!
If you are an overly narcissistic person, then yes, believe in reason. Reason, at best, is highly subjective and will bend to the demands of its beholder. There is no universal banner of Reason flying over our heads, so Reason pretty much enables anyone to do whatever they want, if they can somehow find some "good" rationale behind it using their reasoning skills.
In order to make a solid starting point for a belief system one must always take the most logical approach , one that is devoid of reasons based on subjective viewpoints. Unless you are able to analyze all possible sides of an argument and find that no side is biased your belief system is flawed due to its very subjective nature. For example the belief that blue is the best color is subjective by its very nature and thus a flawed belief for there is no way to prove blue is the best color unless you make up a way to prove it based on personal reason which would be derived from an individual opinion thus leading back to it being subjective. In turn you end up with a belief that blue is the best color by reason of I like blue.