How many Soviet republics were there?

But what about faith, for the Tsar?

Oh, come on. It's all just slogans, pretty words, and nothing more. Few people actually believe in them. It's exactly the same stereotype as "For Stalin!" on the lips of dying Soviet soldiers. In reality, I listed only a small part of the religions that no one ever hindered in the Empire, and the list can be expanded.

And atheism is not a religion.

In the form it took in the Soviet Union, it was a very real religion and even fully satisfies your arguments about "one person believed, forced others to believe...", even more so than Orthodoxy, since it has a rather noticeable division of opinions on many issues, which is not observed at all in categorical atheism.

A person must cultivate culture within themselves, and religion should not dictate to them.

Religion is the backbone, the minimum set of laws for a believing person. And then it can be enriched with anything, choose any music, type of painting, and literature to your liking, develop culturally and intellectually as much as you like.

And where did this Empire go?

And where did the USSR go? And how long did it last?

The Empire was united only by the threat from outside.

We never had any problems with that.

When the revolution had already been living in the minds of the intelligentsia since the 19th century.

The intelligentsia is different. And I must say that under Stalin, the very intelligentsia that dreamed of realizing the revolution, socialism, and then communism, was simply taken and... On the one hand, you feel sorry for these intelligent people, but on the other hand, you understand that everything in this world has consequences.