Someone had a private idea about God
Or someone had a private dream about God
Or someone had a private encounter with an angel
Then that single someone told the rest of the world
This makes other religions impossible to verify because there are no eye-witnesses of the prime event.
BUT how Christianity started:
Jesus spent 3 years doing miracles and teaching – publicly
Jesus was executed – publicly
Jesus was buried and rose from a public tomb – publicly
Jesus showed that he was alive – publicly
Then it was the public that told the rest of the world
Christianity is the world’s most testable religion.