The most wonderful discovery made by science is the very thing that Christians in the USA are most opposed, the fact that we are all connected in the most profound ways. From the particles that make up your body to your DNA, everything is part of a great chemical reaction called the universe, and we are all an integral part of this.
The reason they deny this is because it removes the necessity of a god, ironically it does not remove the possibility. This frightens them because Christians, like the other Abrahamic religions, are invested in a life after death, so they have given up trying to make this one better in the false hope of a reward after death.
Having invested nothing in this life, anything that makes a life after this one less likely (especially the one they believe in) makes them realize just how much of this life they have wasted preparing for something they don't even know exists. We call this type of denial cognitive dissonance, but to make it easier to understand it is better described as a fear of wasting your life on a false hope.
The notion of a life after this one is not without merit, back when we did not have the time and resources to improve our lot it kept us from giving up. But it is inherently dangerous, as we can see with all religions who claim there is one.
First, if you waste this life investing in a future that may not happen you are wasting the one thing you know you have, this life is real, you know it's real, but anything beyond is only speculation. So you are wasting your real life on a speculation, one of thousands such speculations have been made and only a few actually demand investment anyway.
As horrible as this is, it's not even the worst problem with believing in a life after, suicidal encouragement is the worst part. Not content with this life, one who believes there is a "better" life after will be more willing to end this one, to hurry to the better life based on nothing more than speculation and wishful thinking.
This is where atheists or reality based religions differ. We have no choice but to invest our efforts in improving our lot for this life, it is all we know we have.
Religious people will often demonstrate their complete lack of rationality and logic at this notion, saying crimes are the inevitable result of not having a promise of an after life, they also display their sociopathic and psychotic personality traits with these claims as they need eternal reward to not kill, rape, steal, or torture other people.
Yet humans are not the only life on this planet, nor are we the most important, so even religion does not prevent these people from harming us all. Enter the "no afterlife" world view, because we need no reward to invest in our future.
Humanists have figured out one important fact, we will eventually conquer death, but to be there for that you either have to be such a good person that enough people want you back, or you have to help society get there faster. This is a true eternal reward, a second chance that we know will happen someday, likely in my own lifetime now.
But this is not even the best reason to be a good person, the main reason is that when you hurt others you ultimately increase the chances of you being hurt back, the inverse is also true, the more you help the higher the chances of you being helped in return. Many people call this karma, but the real name for it is herd immunity.
While herd immunity is often cited for vaccines, it applies to everything and encompasses the entirety of the world, even the other animals. We need other animals, even the plants, to survive as a species.
This means we must take care of the entire planet and do as little damage to it, lest we drive ourselves extinct for the arrogance of thinking that we are special and rewarded for believing instead of actually being good.