Good question. I'd be a fool to say that I am 100% certain there is no God. What I can say with 100% certainty is that the God most of us were taught about does not exist. The Bible, both testaments, if the ramifications weren't so sad, would be the most hilarious book ever written. The other religious foundational texts aren't much better. Most are very long, poorly constructed farces with the human race as the object of the jokes or wrath of these powerful intervening beings.
I would remind you that there is no such thing as proof of a negative. So, from what we know there is no positive evidence anywhere for a God, gods, demi-gods, etc. That being the case, it's not likely there is one pending some scientific, positive proof of God.
As for me, if there is a God and it in anyway is an intervening God as described in religious texts, I'm putting the useless bastard on trial for negligence and fraud. So even if I'm wrong, no fear.
I am 99.999% sure that we're not the creation of some colossal alien lab student.
However 100% sure that there is no god.
Remember, when you talk of god, you talk of divinity...which is 100% human fabrication. The term divinity isn't even clear to the religious people themselves! It's to do with.. like... good supernatural energy... or something.
And if you're talking of none-divine creators, then you're talking about aliens.
As long as you equally say that it's "unlikely" there is a Santa, or unicorns, or an Easter Bunny... then you'd be consistent. But would you really want to lead people on by pretending that there is such a possibility? I've never raised infants, but if I did, I would not teach such possibilities to my children. It's kinda like saying that snake oils "might" work.
The problem of the question is the definition of the term "god". Some liberal religious people believe that "god" is not a being, but more a feeling they have during rituals or social interactions, even music.
If you define "god" as such a manifestation of a feeling, you could prove the existence of "god" by measuring the brain activity of such a liberal religious person.
Such a definition of the term "god" is used by liberal people who accept scientific facts but want to hold on to their tradition. I think the definition of "god" is one of the major problems for modern, liberal theologians.
Because the term is so badly defined, it is hard to discuss it properly. (That may be the reason, why so many of our atheist arguments focus on extreme definitions of the term.)
I define "god" as a supernatural being or process of some sort. I am totally convinced that nothing "supernatural" exists, whatever it may be. Supernatural means, it is beyond natural processes and out of the reach of science. Rational argumants can only disprove predictions, that are made by religious world views. They can show that natural (!) processes lead to certain phenomena. This can disprove a "supernatural" theory indirectly. A direct disprove of a supernatural idea is literally impossible, because it is out of the reach of any rational method that could prove or disprove.
Therfore we can't be sure 100%.
I don't agree.
Something out of reach from any rational analysis, would be something that we couldn't see, hear, taste, smell, or detect in any way...
In other words, we would have absolutely no reason to believe in it. (The teapot that orbits saturn falls into this category).
But god does not fit in this category... every single religious scripture affirms that god interacted with our "natural" world, so he is not out of reach of any rationality. Simply seeing him, or one of his miracles, would be a rational method of proving him.
However, it is a fact that divinity, god, miracles and related terms, have all been invented at a time where we had no idea of how the natural world worked.
Now we know better, and these ancient terms simply have no more reason to exist.