I like the idea of inverting the "flat earth" model. Theologians argue that the evidence implies a spherical earth, but it's obviously, scientifically, flat. That's awesome.
It really comes down to what you mean by "flat."
If you mean a classical flat plane world, with a dome sky (like those old pictures), the easiest explanation is magic did it.
The creators didn't want humanity looking off into the distance under the sky and seeing the walls of the dome actually reaching down to the ground. To achieve this, they've slightly modified the physics of the world: physical light generally curves "down". Maybe light is affected by gravity at a much higher rate, or maybe the "ether" causes light to slow and "drop" like a canon ball through the air. In either case, sight lines would naturally be constrained to local space.
The stars, sun, and moon in the sky still "work" because their light streams down; there's no more "down" for the light to stream. Sunrises and sunsets are explained by the "tired light" struggling to reach the viewer.
Spyglasses still work too, but nobody knows why. The reason is that everything emitting light emits it in a "spray," and the optics in spyglasses allow you to see fainter light. So...
Note an interesting effect of "light spray." The more powerful the glass, the more "spray" you can collect and see a farther distance... but you can ALSO estimate distance based on the angle offset you have to look "up" to see a light source!
Another interesting effect would be that artillery and archery become even more of a specialist skill, as sight lines are no longer straight, which means that operators will need to use their judgement to offset shots not only for gravity and windage, but ALSO for the optical illusions of range. Magical weapons, perhaps, may have been created specifically to alleviate the problems of range. Or, perhaps, nobody bothers with long range weaponry, and all battles are fought with swords and fists.
Now, again, the point here was for the Gods to hide the seam between the sky-dome and the flat ground, so you've got a couple of options as you walk away from the center of your landmass.
The first is to pull a Truman Show, and have the skydome come down into the ocean. As long as nobody sails into the wall, nobody will know it's there.
Another option is to have the strength of the "curve down of light" increase as you move farther away from the center. As you get closer to the "edge" of the world, the world gradually gets darker and darker, as light emitted gets "pulled" or "dropped" to the ground faster and faster, until by the time you reach the wall it's literally pitch black (all light sources immediately fall to the ground). Combine that with a human fear of the dark, and you could easily discourage people from going too far.