It's not that I am so very sure of God's nonexistence, rather I am unmoved by the arguments, professions of faith, unpersuasive testimony of so-called miracles of the Spirit, and so on and so on. I'm not persuaded. At. All. Not. Even. A. Little. Bit.
My lack of belief should not be construed to mean that I am (hypocritically) certain of God's nonexistence. A seemingly endless parade of arguments each failing to persuade in a nauseatingly similar way does make for low expectations that the next argument will be "the one." Being unable to articulate the exact failures in logic of this particular ploy to save this poor sinner from eternal damnation is not proof of much of anything. Persuasion is not about truth or truthfulness. It's about coaxing and cajoling assent. Assent can be given for many reasons besides concern with truth: to join the "right church";
This bad faith underlies all proselytizing. It is a resolute refusal to acknowledge the emotional and social investment these would-be saviors of Mankind have in their articles of faith. When I came out of my religious crisis "the wrong way," I lost nearly all of my friends. I lost a great deal of support at a difficult time time in my life—I'd just received an academic dismissal. The implied threat of loss makes for extra fervor in attempts to persuade others to one's doctrines. Every soul, Jewish ones especially, bagged for Christ confirms the validity of one's convictions and the legitimacy of this whole approach to truth and persuasion. Each conversion eases doubts and uncertainties for a time, like a drug. In some cases it's certainly a petty exercise of power over another [I have to include this one. I am Nietzschean, after all.]
Early on, I realized that in giving up God and Salvation I had no right to certainty. I found myself unable to discern a difference between faith and mulish conviction. Certainty was just an attitude toward certain ideas and beliefs. It does not touch upon the truth or falsity of ideas and beliefs. Nietzsche's living life as a series of little experiments struck a chord. I resolved to see how it would go living with degrees and shades of certainty. I knew better than to discuss this with the friends I had at the time: they were already upset with me because of coming out of my religious crisis "the wrong way."
Now, some three and a half decades later, I can safely say that it's not impossible and requires attention to detail coupled with fastidiousness in saying what I mean. Not for their benefit. We hear and see only so much. Some more, some less. The limits of our language is the limit of our world. Some of us live in bigger worlds and some in smaller.
Consistently declining to be persuaded by bad arguments is as much "proof" of faith and certainty as is a parent answering a bored child's "Are we there yet? Are we there yet? Are we there yet?" And if there's a lack of certainty and faith in unpersuasive arguments is secularism or atheism a religion like Christianity?
That self-described atheists organize around and pursue various joint projects that cut into traditionally Christian domains confuses these issues because it looks like membership in various secular/atheist organizations presumes certain beliefs as a prerequisite for membership, instead of recognizing that a lack of faith in conviction allows for other possibilities of social organization. It doesn't help that many nonbelievers are themselves confused and inconsistent in these matters. It's a rough thing to come to terms with, but wildly liberating.
To sum up, consistent failure to be persuaded of somebody's doctrines is not prove of any conviction in the falsity of anybody's doctrines. This is a misunderstanding rooted in ignorance. For many, many Christians the beliefs and valuations that they hold of non-Christians is an absolutely essential and irreplaceable part of their religion.
This second point is far, far more troubling than simple ignorance. It is easy to dismiss Christians vociferously protesting Marriage Equality as homophobic bigots who'll get over it. The fact remains that Christianity (but not only) has a long history of doctrines and beliefs about those outside the local congregation stretching back through the Middle Ages on into antiquity.
This irreconcilably conflicts with secularism's ideal of treating all minorities, all religions, all social groupings fairly and equitably. To think otherwise is to indulge in fantasies of squared circles. It should be noted that failed attempts to square the circle led to all sorts of developments in mathematics. In the sublunar world all things are mortal. It would take some time for the pursuit of the ideal to prove itself as fruitless. In the meantime, efforts to reconcile the irreconcilable would exercise creativity, passion and imagination. Faith in a final solution to a problem may be necessary for many people in order to make any attempt at a solution. And if the ideal solution is ever attained, it becomes either worthless or the goal posts are moved.