After listening to lectures from religious believers and talking to them online, I've noticed that they tend to view their God the same way a toddler views a father figure. Daddy is infinitely strong. Daddy knows everything, and he is always right. If Daddy says it's good, it's good. Pointing out flaws and inefficiencies in God's alleged work, and the moral and logical errors in God's alleged plan, seldom phases a believer, just like a toddler can never be convinced that their father is anything less than perfect regardless of his past deeds.
Now let's look at the most important question we all face: What is the meaning of life? It's a scary question, since nobody wants to find out on their death bed that they've wasted their life. After conversing with religious people, I now have some understanding of why the answer seems so simple to them. Do what Daddy says and all will be right with the world. They're happy to pawn the tough question off onto religion and see no reason to contemplate it further.
However, if you do contemplate it further, you'll notice a problem. Let's assume (despite mountains of evidence to the contrary) that God does exist. Let's imagine that cosmologists come forward one day and tell us that they found a large man in space, who has presented compelling evidence that he indeed created the universe, and tells us that he has a plan for us.
How would this plan affect our meaning to life? As thinking beings, we could evaluate this plan, decide what seems right based on internal consistency, how selfish or selfless of God's motives are, how following God's plan would affect other people, the rewards for following God, etc. Notice that even in this religiously ideal case, we're still making judgments based on our own belief systems and values. We're still creating our own meaning.
To counter this point, some Christians will assert that we only have values (and morality, since it follows directly from our values) because God encoded them into us in the first place. This is one instance of religious stupidity that I find genuinely insulting. It implies that my values aren't really my own, and that God merely whispered them into my ear at birth and deceptively* made them feel like my own ideas. My personal revulsion aside, this lunacy runs into an immediate logical problem. If God covertly programmed us to have the belief systems, values, and preferences of his choosing, that is glorified brainwashing, and free will goes out the window.
Contrary to what Christians would like you to think, the bible doesn't actually say that God magnetized our moral compasses or instilled within us our values. However, there are plenty of passages that claim nearly the opposite: that our (God-created) bodies urge us to pursue that which God himself detests. Go figure. Genesis 6:5 Romans 8:5-8 Job 15:14-16 Romans 3:10-18
The crux of the matter is this: in order for anything to have value, it has to be valuable to someone. The idea that intrinsic meaning was woven into the fabric of the universe by a deity, independently of anyone's thoughts, feelings, and values, isn't just impossible; it's absurd. To accept this religious view would require us to reevaluate our most joyous and treasured moments as meaningless, if only we found that there was no deity floating around in space somewhere to reassure us that our joy was real.
In closing, I'd like to add that not only does nihilism not follow from atheism; it is religious fanaticism that is the true nihilistic position. If your interests all revolve around pleasing an invisible (and in all likelihood nonexistent) worship-hungry narcissist in the sky, and all of your good deeds are done to achieve this end, then you don't really care about the people in your life or anything else in the real world. That is the epitome of nihilism.