“How did people know about God before the Bible?”
Good question. Most of human history was a Bible-free zone.
The Bible as we know it — the 66 books with which Protestants are most familiar — is actually a relatively new thing, which is probably what the writer of today’s question is getting at. Roughly speaking, the individual books found in our Bibles today were written over a 1,600 year period beginning about 3,500 years ago, which means almost half the history our Bibles record took place millennia before anything “official” was done to preserve it.