How do we know? fossils. stuff that has been around a long time, is durable, and looking at the other fossil, light. Yes, light, is a fossil, as old as 13 billion years, it carries the imprint of what it reflected off.
Everything we see , we see from the past, one way or another. fossils.
No, God did not tell them, otherwise they would not have copied mistakes like Gen. 1:1 asserting that the Earth was created in the Big Beginning. Or that a wooden ship could be build big enough to float in water that falls at a rate of meters per hour.
priests wrote to get the 10% phoney fee.