The United States is the world's largest economy according to nominal GDP. It's the world's largest democracy, and by far, the world's largest military force. It is capable of destroying the world many times over, but it thinks of itself as the world's protector; a beacon of prosperity, democracy, and liberty.
To a large extent, the US is those things. US citizens are prosperous, not because the country would sit on an abundance of natural resources, sell them to the rest of the world, and do nothing else; but because the country employs an economic system its participants can depend upon - a system that emphasizes honesty, consistency, dependability, and responsibility. As opposed to many places, the US system creates an environment where risk-taking is possible, and rewarded generously. The result has been a prosperous economy that makes even poor US people richer than middle-income people in many countries.
The US is also relatively free. It might not be, socially, the free-est country anywhere in the world; but it's up there. It's large enough, and free enough, that if you dislike social mores in your environment, you can move across the continent to a state that's more to your liking.
The US has issues on which there's polarizing public discourse; such as the role of money in politics, military adventurism, the health care system, and others.
But what I want to talk about is this.
Evolution is the only accepted scientific theory about how humans came to be. It implies that human biological lineage is hundreds of millions of years old, and dates back to very simple organisms in ancient oceans. Its main opponent is the creationist hypothesis, which implies that the Universe was created as recently as 6,000 years ago. This hypothesis is believed largely by people who subscribe to an Abrahamic religion, and is spread mainly by them.
Now, religion specializes in claims that are unverifiable on purpose. But over time, the domain of science expands, and it becomes possible to disprove some religious claims. Religious beliefs are benign as long as they remain restricted to claims that can't be verified. But the problem is, they don't remain so restricted. The inclination towards blind faith is harmful when it encourages belief in claims that have been conclusively disproved.
The problem, then, is that up to 60% of the US population are being incapacitated by religion to such an extent that they reject, or do not accept, one of the most important bodies of scientific knowledge, backed by mountains of empirical evidence, which anyone who sets their mind to it can validate and reproduce.
So why is this worrying?
Well, it's worrying because it's a departure from reality; and when the human mind departs from reality, reality does not follow with it. When your behavior and expectations are at odds with how nature works, you're going to be surprised in all sorts of unexpected and undesirable ways. The people who believe these things also believe in restrictive social mores which are not founded in evidence, make little to no sense, and tend to punish draconically for harmless transgressions.
When a plurality, perhaps even the majority, of population are this unreasonable, the very fact that they have voting control over the world's largest democracy, it's largest economy, and it's largest military, is worrying.
In United Arab Emirates, today, kissing in a taxi can get you a year in prison. The US is already not far from that. Possession of a harmless plant lands you in jail. Many people with largely innocent transgressions are being added to sex offender registries for life. Religion is the main uniting force through which this intolerance spreads, and disbelief in evolution is the dead canary in the coal mine.
Religious belief is not harmless. Reasonable people need to fight against it, or else that's the end of liberty, for everyone.