And you "know" this because?
A. You have overcome what the corrupt world teaches and have carefully examined all the facts, including Church Fathers and what the Bible says about the Firmament.
or
B. You believe what you have been taught to accept by the corrupt "educational" system, the corrupt media, and deep down you fear of being mocked like those of us who have done "A" and have reluctantly concluded that the globe earth spinning earth spinning around the sun spinning around in the galaxy is a hoax.
The earth indeed does appear to be flat. And we do not fall off it. Which at first glance makes it feel like the earth is flat.
But, then there are the other heavenly bodies around us that are round such as the moon and the sun. What are the chances that this Earth is round too?
Then again there is something called gravity. There is something that returns the apple to the ground after you throw it in the air. And if the earth was round then that explains why we don't fall of it. Because gravity keeps us there. Unless we defy it.
The Ancient Greeks guessed the earth was round, so it's not a new concept. And they believed in the force of nature which kept us on the ground.
The descriptions of the Bible do not contradict this. It is written from the perspective of a man staring out into the heavens and conveying the mystical cosmos. The firmament is the expanse or the atmosphere.
Neither does heliocentrism contradict the Bible. We live in a Solar System with the Sun at its center. Again, the Ancient Greeks guessed this. The descriptions in the Bible are from the perspective of man living on earth, rather than the geometrical center.
If Almighty God had intended Holy Scripture to be a series of scientific treatises it would not be the highly engaging work it is. But Providence made a masterpiece that speaks to all men from all walks of life.