They were/are secular, meaning they have no official role for religion in governance (and you can add France and Turkey to the list). That’s very different from atheist states, where religion is officially not tolerated (or banned).
There’s too many other reasons (ie economic differences) that make such comparisons impossible.
Darwinism and science aren’t really religious beliefs, and they’re not things that people would have as the main principle of their lives. First, ‘Darwinist’ is a term that is often attributed by conservative religious people to those who ‘believe in’ evolution. Modern understanding of evolution is very different than Darwin’s understanding. People who support evolution today do not ‘believe in’ Darwin’s ideas in the way that religious people believe in their holy texts. Darwin is simply the first person who published a detailed description of how he thought evolution might work. His ideas are not considered unassailable or universally correct, nor is he worshipped in any sense. Really, the idea of ‘Darwinism’ is a result of imposing a religious paradigm on a non-religious concept – it is the assumption that since evolution is an idea which people apparently ‘believe in’, they must have a sacred book and a holy person to turn to. It’s just not like that.
Second, science is not something that’s suitable to use as a guiding principle in one’s life. Perhaps the scientific method of forming hypotheses and adjusting them over time to conform with experimental observation is a valid guiding principle. However, science as a means of obtaining objective knowledge about material fact by empirical observation is simply not capable of answering many of the questions religions claim to answer. There can be no materially true philosophy or ethics, since these are inherently subjective. There can be no way of determining if there is life after death or if there is an all-knowing, all-powerful entity in the universe, because these are not empirically observable.
People who are not religious do not have some quasi-religious thing that plays the role religion does for believers. They may have guiding principles, but they do not have, for example, sacred books or ceremonies or ideas about the supernatural which they know to be true. They may consider many aspects of religion unnecessary for leading a meaningful life, and they do not need something to replace these aspects.
I should say, though, that this only applies to people who have decided for themselves to be non-religious. For people who have had their religion taken from them, as in the USSR or China or Nazi Germany, the experience can be devastating. They may continue to practice their religion in secret, or many may return to their old faith once it’s legalized, as happened in the USSR. The same thing applies, of course, when people have a particular religion forced upon them, as happened in Afghanistan. The religious makeup of a given country may change as its culture does, but it has to be a natural process. State interference in religious issues generally isn’t a positive thing, or a good thing for the people involved. Oh, and vice versa.
It always seems to me that religion takes the biggest back seat in people’s lives in those European countries that are most protestant - certainly not France or Turkey, whatever the official policy. So I’d be thinking Sweden, for example, which I think is mainly Lutheran, but where the dominant mind set appears to be secular. Could be totally wrong, prepared to be flamed by a Swede.
I’m also thinking Iceland for some reason.