Over time I've seen arguments come and go, and even from the seemingly intelligent, attempt to argue in favour of, or at least rationally explain, theism and fail miserably.  As goes the saying goes, if you want something done right, you have to do it yourself.  Despite not believing in their assertions, I can certainly do a much better job arguing for it.

As with anything complex, you must address it from the ground up.  I will lay out the parameters for which the construct of theism can be reached as a rational conclusion.

1. Degree of certainty.


2. The existence of supernaturalism


3. The existence of a god or gods


4. Personal interaction with a god or gods
   

Before we can even begin with investigating the concept, we must set a goal.  In order to suggest practical subscription to a belief, it must only be established it be reasonable to be sure to a degree greater than 50%.  However slightly, as long as it's above 50%, you generally believe the notion to be true.  To that end, the goal isn't to establish certainty by any means, but decent enough likelihood to establish expectation of factuality if the answer were exposed.

To address the question, we need to evaluate whether something beyond the natural universe exists at all.  This can be simplified into a philosophical evalution for which no evidence can be provided: The question of why; not in a how sense, but purposeful sense, even if it's not a specific job for you to do in life.  Why is there or should there be existence at all?  Regardless of life, just matter or spacetime.  If you accept that the question as valid, you can then take the position that existence is not self-substantive.  The obvious refutation is rejecting the question in the first place.  But what would be the purpose?  If I were personally going to offer one, it would be existence itself.

In terms of a god or gods, the most important point to address is Richard Dawkin's Ultimate 747, wherein a god is more complex than the question for which it is meant to explain.  Impressively enough, he also includes the response: that the god, if it were to exist, it must still be the process of building from simplicity.  While this dismisses religious claims, theism itself in concept can stand as unrefuted--spiritual existence by naturalistic means in that spiritual realm may still exist.  I cannot though offer argument in terms of a god of creation, only logically that in a extra-universal world with extended existence that there be beings that are both supernatural and beyond human ability and in that definition godly.

The last is a personal interaction.  Theism by definition does not require this, but the concept is philosophically used to describe belief in a personal god as opposed to deism which describes belief a unobstrusive god.  Logically speaking, the concept of all-seeing, all-knowing and all-powerful are all individually self-refuting.  What would be more logical in our concept of merely supernatural superhumans is a personal interest of a god or gods in few.  Additionally, miracles of any significance other than to the mentality of the person in question would be utterly destructive of the naturalistic requirements of an existence.

So, where do we stand after this?  Well, the why question is an arbitrary decision one may make a decision on, that I would consider unreasonable to come to a 'yes' conclusion on, but could not give a reasonable argument to.  If all you believe is a supernatural existence of some kind, I can not dismiss your reasons outright.

On the matter of a god, it is merely that no one can tell you it's impossible.  There is no argument actually that favours it, only that no argument can outright reject it, either.

And on personal interaction, it simply adding a characteristic to a completely arbitrary construct.

Of course my arguments aren't convincing, but they are honestly the best I can establish having considered the matter without exaggeration since I was six years old.  The only thing that in the past led me to the mere belief in the possibility of afterlife at all was the question of existence itself, which I leave to you as the only inarguable point that is a philosophical decider -- either you accept the question or you don't.

And in case someone thinks I didn't want an answer, all such evaluation was spurred by an, in its only practical use of the term I can fairly use, awesome fear of death.  If you think that isn't reason enough for utter desire for a conforting answer, you have not felt that fear.

I reject the arguments above because they are unconvincing, and for the claim, essentially speculation in lieu of the necessary extraordinary evidence.