This is something that I have struggled with, and I am sill not certain whether I more partial to the nihilist atheism, or the existence of a God.
Okay, let's assume atheism-nihilism is true. If nihilism is true, then where would that leave you? If nihilism is true, then there is no ultimate moral value, meaning, or purpose in life. There are no moral absolutes. In the end, we'll all die. In the end, all life in the entire universe will eventually die. Nothing really matters in the end.
At best, you can try to create subjective moral value, meaning, and purpose for yourself. You can believe or do whatever you want with your life so long as you're happy. However, the catch is this is equally true for everyone else. What I mean is, if nihilism is true, then, sure, someone can choose to live like a saint because that's what makes them happy. However, another person can just as legitimately choose to live as a serial killer if nihilism is true. Again, there is no ultimate moral right or wrong or meaning or purpose in life if nihilism is true. Hence, if a person enjoys murdering people for sport, and can get away with murdering people for sport, then there's no objective reason he or she can't murder people for sport if nihilism is true. Like it or not, this is the logical consequence of nihilism.
Sometimes a picture (or a short movie!) is worth a thousand words. Simply watch the (free) short film "Cruel Logic" if you wish to see an example of nihilism taken to its logical conclusion.
However, the problem is many nihilists are sort of hypocrital about their nihilism. Or at least they won't accept the logical consequences of nihilism. On the one hand, they'll say: life is meaningless! But on the other hand, they'll say: but I'm going to tell you how to live! However, if life is ultimately meaningless, then no one can tell anyone else how they should or should not live.
If nihilism is true, then humans are no better or worse than any other organism. If nihilism is true, then human life is ultimatey valueless. If nihilism is true, human life is ultimately worthless. Suppose there is a superior intelligent alien species like in the Alien movies or the Predator movies. If such aliens exist, then they might regard human beings like how we regard insects. They might regard humans as prey or game to hunt like how we hunt deer. Just as there's nothing immoral about humans killing insects, likewise predatory aliens could kill humans with zero moral qualms if nihilism is true. In other words, what I'm saying is, if nihilism is true, then humans don't matter.
If nihilism is true, then what fundamentally matters is who is the strongest or most powerful. The one with the most power is the one who will be able to squash others. Hence, if nihilism is true, then there's nothing objectively immoral with radical Muslim jihadis like ISIS or Mexican drug cartels like Los Zetas who terrorize people, torture people, murder people, behead people, including women and even little children. If nihilism is true, then you can't say ISIS or Los Zetas is doing is wrong in an absolute moral sense.
At most, you can say you disagree or you dislike what they are doing, but they could turn the tables on you and say they disagree or dislike what you're doing! They could say they dislike or disagree with living according to 21st century liberal western values! That's because groups like ISIS and Los Zetas regard 21st century liberal western values as weak and effiminate. Instead, they wish to live like warriors because their core values or ethic is the ethic of a warrior culture. In this respect, they're no different from the Nazis, the Imperial Japanese, the Mongols, the Huns, the ancient Persians who practiced a horrible torture known as scaphism where victims were tied and trapped inside a little boat like a canoe so that they couldn't move, then honey poured on top of their torso or abdomen, and vermin like insects eat their way through the victim who is still alive. Yet, if nihilism is true, then this isn't absolutely morally wrong because there are no moral absolutes. Again, if nihilism is true, then all that matters is who is in power, who is the strongest, hence Nietzsche's "will to power" where "might makes right". The ones with the most might are the ones who decide what is right.
As far as science is concerned, if nihilism is true, then there's no value or meaning or purpose in trying to explore and discover more about the world, which is what the scientific endeavor is all about at heart. Sure, you can investigate the world and its many marvelous phenomena if that's what makes you happy. But just as well another person could live their life without a care about what anything means or how anything works. If nihilism is true, then investigating the world is not superior to not investigating the world. It's just about what value or purpose or meaning you wish to make for yourself. Hence, if nihilism is true, there's no substantive rationale to explore the world. In this respect, nihilism is anti-scientific.
If I were to decide that the existence of a God is the most logical explanation however, how would I know that he is the Christian God? I like the Christian religion the most (probably because of my background), but naturalistic theology doesn't seem to necessitate the existence of a Christian God.
The existence of a camp fire doesn't "necessitate" the existence of a person lighting the camp fire per se, it could have accidentally sparked and ignited on its own, but that doesn't imply there isn't a person who lit the camp fire either. Similarly, the existence of the universe doesn't "necessitate" the existence of God per se, but that doesn't imply God doesn't exist. After all, the question still remains: why is there something rather than nothing?
I think one of the better popular level books I've read arguing for the Christian God is Why Should I Believe Christianity? (James Anderson). It's a short book, only 200 or so pages, fairly cheap - only $10 or $15, and written by a British professor with two PhDs, one in computer science and the other in philosophical theology, if I recall correctly. He's also a Christian and a Calvinist. I can try to discuss specific questions if you have any too.