"If Christianity is false and a worldview containing atheism is true, then nihilism is logically entailed."
Someone asked me to defend the above. So, here it goes. By the way, I've provided an adorable little dictionary at the end to help clarify my points.
I. If God does not exist, then all human life as well as every individual life will ultimately be destroyed.
II. If there is no God and no life beyond the grave, then life itself has no ultimate meaning, value, or purpose.
A. Meaning 1. Without immortality your life has no ultimate significance and makes no difference in the world’s outcome. 2. Without God there is no broader framework within which human life can be seen to matter. The universe isn't capable of giving meaning. It's just there. Only humans, then, can give ourselves and each other meaning and that is hardly ultimate.
B. Value 1. Without immortality there is ultimately no moral accountability, and your moral choices become inconsequential in the end. Only that which is subjective stands but is inadequate to demonstrate that human life is ultimately a good thing.
C. Purpose 1. Without immortality your only destination is extinction in death. 2. Without God there is no ultimate purpose for which you came into this world. Any purpose would be, perhaps, the whim of your parents. For some people, not even that much.
III. It is impossible to live consistently and happily with an atheistic worldview.
A. If we live happily as atheists, it is only by subjectively affirming meaning, value, and purpose for our lives, with a lack of any ultimate foundation for them.
B. If we recognize the above as atheists, we will experience a profound unhappiness and even despair because we know our lives are really meaningless, worthless, and purposeless. (popularly known as existential dread).
If a worldview in which atheism is true corresponds to the actual world and we come to know this truth, then it's logical to reject all religions containing a god. If a worldview in which it is true that there is no immortality and life ends at the grave corresponds with the actual world and we come to know this truth, then it is logical to reject any moral principle that we don't happen to agree with at any particular moment. Unless, perhaps, there are foreseeable negative consequences that we'd prefer to avoid. Then again, perhaps not. If a worldview that contains all of the above is the actual world, then the only meaning we have is the temporary meaning that we assign to ourselves or have assigned to us by others. In other words, our lives are ultimately meaningless.
Therefore, nihilism is logically entailed.
But many will simply reject it because they wanna. And I don't blame them because it's pretty depressing! So, we ought to literally thank God that He exists and has granted us an existence in which it is unjustified to hold to nihilism. We can rest assured that our lives don't merely have some meaning, purpose and value. But, in addition to the finite purpose, meaning and value that I mentioned above; we can be thankful that our lives have ultimate meaning, purpose and value!
Meaning: implied or explicit significance Nihilism: the rejection of all religious and moral principles, in the belief that life is meaningless. Purpose: the reason for which something is done or created or for which something exists. Ultimate: being or happening at the end of a process; final. Value: the regard that something is held to deserve; the importance, worth, or usefulness of something. Also dealing with what is morally good and evil.
P.S. I'd like to give credit to Dr William Lane Craig for this insight and format. You can find this information provided in his book, On Guard.
The artist of the webcomic can be found here.