Does the "sacred" also exist for atheists and non-religious people?
As I understand the concept, the answer is no.

The "Sacred" is that which, so to speak, protrudes into our world from a sphere that transcends the human and natural world; the Sacred manifests itself in our world, but its origin is outside our world.
This radical transcendence is crucial.

Some scholars, such as Hans Joas, define the "Sacred" as a reaction of a human being, as a human condition, a feeling. The sacred would then be something which seizes and shakes me or us in our innermost being. But this theory is misguided, because often enough the presence of the sacred causes the opposite in believers: inner peace, instead of existential upheaval. Moreover, there are quite mundane things or events that can seize and shake a person (a fatal illness; the death of a loved one, etc...).

Many non-religious people use the term "sacred" as a synonym for "this is very important to me or to us." For example, a mother might say "the family dinner together is sacred to us".
Now there is a simple litmus test to find out if this is really something sacred or just something very important: Try negotiating with the person in question about what is "sacred." For example, you offer that family mother 10,000 dollars if she skips family dinner just once. Most likely, she will accept the deal. If not, then you could raise the price: How about a 20,000 dollars?

And then we try to make the same deal with a deeply devout Catholic: Let's offer her 10,000 dollars for not swallowing the consecrated host at the next communion, but spitting it on the floor and stepping on it. You can imagine the reaction of the person. Things would not be better - rather even worse - if we tried to increase the reward for the desecration of the host. Would she do it for 50,000 bucks? She would probably spit in our faces, "I'm no Judas!"

This is the decisive difference: the sacred comes from another sphere, withdrawn from human influence, and therefore someone, if he or she believes in the existence of this sacred sphere, cannot negotiate about it at all. On the other hand, everyone can freely negotiate what is important to him or her. The sacred is simply not negotiable.

And therefore the sacred exists only for people who believe in such a radically transcendent world or order. For atheists, there are only more or less important things, but nothing sacred (unless, for example, one elevates the nation or the dignity of humankind to the rank of sacred, in which case these are also non-negotiable. In that case, one might be an atheist, but a believer in a non-theistic religion like Nationalism or Humamism).