There has always been a certain inconsistency in preaching atheism. Generally speaking, an idea becomes more important the greater, and more lasting its effects, whereas even a true idea doesn't really matter if it has little to no effect, particularly over the long run.
Christianity, by nature, has eternal effects. If you're a Christian and you're right, you go to Heaven forever. If you're not a Christian and you're wrong, you go to Hell forever. But contrast that with atheism. If it's true, so what? I can't change my eternal fate by converting to atheism, nor can an atheist lose anything by converting to Christianity.
Even the short term effects, those that affect our lives now and the lives of future generations, are irrelevant. The first reason for this is that there is no objective standard for what is "good" and what is "bad". Even atheists have different opinions on what is good and bad. Atheism is not a codified religion, but rather it is a religious orientation, leaving the individual believer to accept whatever moral code they desire (or none). Therefore, any short term effect of atheism is completely pointless.
But the second reason it's irrelevant is that even if all of humanity agreed that the natural effects of atheism are "good", none of them will remember it, nor will any of them be remembered. A time will come, whether in one year or billions, when conscious life in the universe will cease to exist. Eventually, the universe itself will suffer what is called a "heat death" (i.e. when all the matter and energy in the universe has reached its most probable distribution). Ultimately, that means you gain nothing, you lose nothing, no matter what you believe.
Consider a colony of ants. Do you care what happens to them? They might, but most people aren't opposed to pouring a freshly boiled kettle into their nest, painfully killing most of them, seriously injuring most of the survivors and generally devastating the entire colony. But to you, it's irrelevant. Death is inevitable, suffering is natural, and ultimately the universe is as indifferent to you as you are to those little ants. Ultimately, therefore, it's all a matter of perspective. Subjective opinions that are as meaningless to anyone but the individual believer as one's favorite ice cream flavor.
So why preach atheism? If it's true, it's irrelevant. If it's not, it's dangerous. The logical conclusion to this is that there is absolutely no good reason to preach atheism.
"But Christianity isn't the only alternative!"
I know. I never said it was. However, it is objectively more logical to preach Christianity than it is to preach atheism. If atheism were true, it would be illogical to preach it for the reasons stated above, but it would not be illogical to preach Christianity. If a Christian dies and it turns out he's wrong, what does he lose? His ultimate fate would be exactly the same as if he was an atheist, and all the other atheists would also share the exact same fate. An eternal non-existence. No regrets. No loss from Christianity, no gain from atheism. If Christianity is true, it is infinitely more logical to preach it, because not only is it true, it matters. Even if some other religion is true, it still makes sense for Christians to preach Christianity because no matter which other religion is true, Christianity, by its very nature, still requires its followers to proselytise, and still believes in the eternal consequences of its acceptance or lack thereof. Basically, Christian evangelism is internally consistent, whereas atheistic evangelism is not.
Now, all of this isn't to say that atheists shouldn't preach. Since atheism has no moral values, you can preach that the sun is made of cheese and farts out skittles, it has as few moral implications as lying to defend something like Evolution. But regardless of whether or not it is morally consistent to preach atheism, it is not logically consistent to preach atheism. This makes Christianity the objectively more logical topic of discussion. Unlike atheists, we have reason to preach.