Why would one feel existential dread? I am the same person as before? The idea of dread could come if one believes there is some 'purer' or 'higher' form of existence in comparison to you, it should not come from realizing you and all other lifeforms are made of the same stuff.

This isn’t true. Plenty of people aren’t religious. The explanation for why is more mundane.

It’s because of identity. Because people who are religious spent years and years and most of their lives not only studying and believing what they believe but also building community and centering their behavior around it. Abandoning that is the harder thing to give up.

If it were existential dread then we wouldn’t have entire countries like China being mostly atheist.

Nonsense.