There is, of course, some ultimate reality. The ontological truth is out there. Even if naturalistic atheism were the truth about ultimate reality, this couldn't be objectively known in this lifetime. Ditto for the Christian and Buddhist claims as well, of course.
Individuals can only reach a level of conviction. Those who claim to "know" - typically religious believers - are talking about an "inner knowing" that is really a strong conviction rather than actual knowledge and that carries no particular weight with anyone else. Even if I'm utterly convinced those voices in my head are the Holy Spirit, there is simply no way I can objectively know this.
I believe the effort to reach a strong level of conviction will take into account an individual's experiences, observations, studies, reflection on all the foregoing, and lastly intuition. (Many sages across many traditions have said intuition is the ultimate form of knowledge.) Experiences may or may not include some the individual regards as supernatural or even as revelations from God.
Each individual must decide for himself or herself which areas of study are most relevant and what types of evidence will be allowed and considered. When you examine peoples' belief systems, you often find their convictions were really more like self-fulfilling prophecies. Their focus was so narrow, and what they were willing to consider was so limited, that the end result (be it atheism or theism) could have been predicted from the start.
To me, the threshold issue is deciding between naturalism and "something more" as the ultimate nature of reality. (Even many atheists are in the "something more" camp.) Once you arrive at a level of conviction on this, the next issue is whether atheism, deism or theism has the greater explanatory power for reality as you believe it to be. If your conviction inclines you toward theism, then you must decide which of the theistic alternatives has the greatest explanatory power and seems intuitively to be most likely to be true. You then put your faith in that alternative, and hopefully your conviction will deepen as you live it.
Obviously, most people don't do all this as any sort of formal process. Many - perhaps most - simply cling to the indoctrination they received as children or hold their beliefs more on the basis of cultural conditioning, social or economic considerations, or simply what they find appealing. Many, atheists and theists alike, hold their beliefs almost mindlessly. Others really do engage in a quest.
I had a startling and unexpected born-again experience when I was 19. It triggered my spiritual quest but could just as easily have short-circuited it. I could've simply dived into Christianity and never looked back. I did, however, realize this one experience wasn't enough for the level of conviction I wanted. This was partly due, I'm sure, to the fact my inirial experiences were with a rigidly fundamentalist ministry, and I found myself having to pretend to believe things I deeply questioned.
So I stepped back to square one and engaged, semi-formally, in just the sort of quest I describe above. Perhaps three decades later, I had arrived as a really strong level of conviction and fully accepted my born-again experience for what it had first seemed.
For close to 20 years now, the epistemology of belief has been my primary focus. As you may know, the controversial Reformed Epistemology of Alvin Plantinga requires no evidence at all or any quest whatsoever. Plantinga maintains Christian beliefs can have warrant (i.e., be epistemologically justified) simply on the basis of a rational response to the sense of the divine (sensus divinitatis) all people have and the internal witness of the Holy Spirit. I don't disagree, but I'm more concerned with having strong convictions about my Christian beliefs than simply being able to assert that they aren't irrational.
I just skimmed some of the above posts, but I'm seeing some of the debate that always occurs in discussions of this sort. Naturalistic atheists insist there is "no evidence" for God. They typically insist upon a narrow definition of "evidence" that essentially means "falsifiable scientific evidence" and effectively eliminates human experience and mountains of experiential evidence, not to mention all philosophy and theology. Even science doesn't limit itself to the extent a naturalistic atheist insists theists must do. Naturalistic atheists place themselves in the intellectual straitjacket of philosophical naturalism, whereby theism is simply not allowed. Most of us simply decline to place ourselves in this straitjacket.
I found that scientific evidence from the fields of physics, cosmology, neuroscience, consciousness studies, PSI research and other areas greatly aided me in my quest. No, none of it "proved" the existence of God, but much of it pointed in that direction and was difficult to explain in naturalistic terms. This is the entire point of the Intelligent Design movement - i.e., some really compelling scientific evidence points to a designer as the inference to the best explanation. Those wedded to the paradigm of philosophical naturalism have no alternative but to attempt to shout down ID because a non-naturalistic designer is simply not allowed.
