I've got to nitpick at a couple of things in your opening.
I don't believe that anyone who is a athiest is 100% certain that there is no external force, deity, god, life force, Gaia whatever you want to call it. It is just that the weight of the evidence is against it and continues to be stacked against it.
The evidence might be stacked up against some particular theistic claims, but there isn't really any evidence that there isn't some possible thing we might consider a god. It's simply that there's a lack of evidence even for the broad case for a deity.
For those who are theists the burden of proof must be on them. Particularly for Christians.
I don't agree with the latter point if I'm interpreting it correctly - Christians are not particularly special in having to have the burden of proof. The requirement is basically the same as any other serious theistic claim. The actual things needed for proof might differ, but the requirement does not. The only thing about Christians is that most of us here encounter far more of them than people from other faiths, so we're more likely to focus on it than other religions, but that doesn't mean we disagree with it any more or less than those other faiths.
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Because I believe that having faith in your life leads to overall happiness. I am saddened that some people are so easy to simply quit their faith. It's easy to fall into the trap of secularism and just deny everything you can't see with your eyes. I know that's not necessarily true in your case but for many it is.
1. Having faith might make some people happy, but if what you have faith in isn't the truth then that can have consequences of it's own.
2. Most people who actually have faith who end up giving it up don't do so easily. I don't think you're intentionally doing so, but you're belittling the struggle these people often go through. By coming to the conclusion that their faith is wrong, many have to come to terms with the fact that they now perceive that much of what they did in life was dedicated to something that they now believe to be false. They also have to deal with their family members and friends who are still in the religion. Many feel they need to keep it hidden so they lie to keep their families happy, but it tears them up inside. They fear that if they tell their family the truth that there will be big fallout with those family members, including the possibility of being disowned. (which can and does happen) Because of this kind of thing these people often really, really try to believe, but they just can't bring themselves to because their minds tell them it isn't true.
3. Secularism is not a trap. I resent the very notion. It's very possible to live a happy and fulfilling life when you're secular - many people do, in fact.
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Imagine I am a Muslim. I hold that everything in the Qu'ran is infalliable and true. My evidence for this is... the Qu'ran which tells me everything in the Qu'ran is true. This, as I'm sure you know Jmfsct, is a logical fallacy known as "the argument from authority".
Actually that particular logical fallacy is known as circular logic.
