I have been debating with myself one specific point which essentially stems from the fallacy that a lack of evidence is not an evidence of lack. (Note that this a purely a thought experiment, not a claim, and that, on balance, the whole set of arguments against theism outweigh one possible philosophical issue which only relates to intellectual decency.)
Essentially, the fact that there is a lack of evidence to support theistic views does not mean that it's evidence of a lack of God. Imagine God to be something similar to our 21st century minds and technology like the continent of America was to 15th century Europe.
(Side note: To avoid potential nitpicking about this specific premise, I know that America was discovered and rediscovered, but the "official" discovery was an accident as the goal of the expedition was to find India. That some in the 15th century Europe might even have been aware that America existed is a bit irrelevant as society as a whole certainly did not believe it was there, and it's the mindset of those who did not believe which is interesting.)
Now, try to enter this mindset and think that God is in a similar situation. Just like the 15th century mindset and technology could not comprehend the continent of America, the mindsets and technology today is not capable of providing the evidence required to detect or understand a divinity.
Therefore, a God may exist (but it doesn't matter).
Edit: The title was erroneous in it's use of the word "for". This is not an argument for theisim, it's an argument not against theism. Thanks to Kasu for this insight.
I'd like to defend the statement "Bottom line - ALL gods are man-made."
Unless you count direct divine intervention, which is a silly notion on a number of levels, or the divine being working by some underhanded way in our minds which science can't explain (and never will be able to), then the Gods that are worshipped must be man-made.
Now, this does not mean a divine being can't exist nor that we wouldn't be innately able to understand it. We innately knew complex mathematical models, such as vectoring, which we utlize everytime we throw a moving object (i.e. a rock) at a moving object (i.e. a bird), before we were able to formalize it. But this part just leads to another conundrum, if we can some day scientifically explain a divine being, is it then a divine being?
On the other hand... What if what defines the human experience is the ability to create Gods? I know of no other creature with this ability, and it appears we've been doing it for some time.
Argument for theism: While talking with un-evolved theists they will make the objection that without god there would be no morality. Without god they would be raping, pillaging, and killing. To these folks I say "theism" should be your way, for some the idea of heaven and hell is good and safer for the rest of us. (tongue in cheek)