'Of the first day Moses says that “In the beginning God created heaven and Earth.” And before creating intellectual and rational creatures, desiring also the order of executing these works to be most perfect, He created heaven for angels and men, and the Earth as a place of pilgrimage for mortals. These places are so adapted to their end and so perfect that as David says of them, the heavens publish the glory of the Lord, the firmament and the Earth announce the glory of the work of his hands (Ps.18:2)…. Of the Earth Moses says that it was void, which he does not say of the heavens, for God had created the angels at the instant indicated by the word of Moses: “God said: Let there be light, and light was made.” He speaks here not only of material light, but also of the intellectual or angelic lights….God created the Earth co-jointly with the heavens in order to call into existence hell in its centre; for, at the instant of its creation, there were left in the interior of that globe, spacious and wide cavities, suitable for hell, purgatory and limbo. And in hell was created at the same time material fire and other requisites, which now serve for the punishment of the damned.'  ---  Mary of Agreda: The Mystical City of God.

Mary of Agreda had a lot of good things to say, but do her visions replace and override Scripture?

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The shape of the Earth as seen on the moon during an eclipse is always a full sphere.

The Earth is a sphere, which includes all things above and below the LAND.  Why cannot the LAND itself, be flat?

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I don't think I understand your point. The planet we live in, i.e. the hard mass that we call the earth, is a certain shape. I (and many others) believe the shape to be basically a sphere. There are people here saying it is more like a disc. It can't be both, right?
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If it's possible to view the earth from afar, it has to have some shape, doesn't it?  A sphere, a block, a rectangle?

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Yes. It is spherical. It casts shadows on the moon during a lunar eclipse, from numerous different angles, and the shadow it casts is always round. A sphere is the only shape capable of doing that.
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We know that hell is UNDER the earth and heaven is above it.  Don't these things add to the shape of the "flat land" to create a sphere?  I don't see why not.

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Hell is traditionally believed to be in the center of the earth. In Scripture, in the book of Numbers, I believe, some evil people were destroyed by the earth opening up beneath them and them falling into it. I don't think the Church has an official teaching on this, but it's a pretty common belief.
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Ultimately, "flat earth" is an incorrect term.  We should call it "flat land" theory.

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I don't understand the difference between those two terms?

I don't understand the difference between those two terms?

It tries to maintain the consistencies apparent with evidence for a spherical earth with those finding of adherents to the flat earth. So, the land itself (including the oceans, ice shelf, etc.) make up one concentric disk enclosed within a sort of "glass globe" which is the firmament. The heavens being in the "upper hemisphere" of the firmament, and hell (the underworld) being beneath the land-disk in the "lower hemisphere". Not unlike ancient depictions of the earth:

DigitalLogos, thank you for the pictures.  I'm not a scientist by nature, nor am I very gifted in such studies, by logically, I don't see why a "flat land" and "sphere earth" cannot co-exist.  To me, it seems that the term "flat earth" lacks proper distinctions, which is a curse of modern society.  
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Most *apparent* contradictions can be cured using proper distinctions.

Oh yes, I agree. That's why I'm enjoying this discussion. To me, if we are at all to entertain the idea of a flat earth, we still must take into account those things which are contrary to such a thesis (such as shadows on the moon, et al). 

I start to get lost when trying to talk about eclipses and such.

Here are some videos which explain the FE point of view on that:

https://www.youtube.com/c/DITRH/search?query=eclipse

The firmament makes it confusing. And the waters above the firmament. This makes it seem like the sun and moon cannot be millions of miles away but must be closer to the earth inside the firmament. Then above the sun and moon is the firmament, and above the firmament is vast amounts of water and above the water heaven. The stars and planets are inside the firmament and it is an optical illusion that when we look at them with telescopes they seem like they are far away. Otherwise, where and what is the firmament? The Bible speaks of it. Was that a falsehood? I always pictured the firmament as being a sphere around the round earth above the atmosphere. I have heard prots say that the firmament was destroyed during the great flood, and the water that came down as rain was the waters above the firmament falling down, so now there is no longer a firmament. They also say the sun was above the firmament and the firmament used to protect us from the sun's radiation, which is why people used to live to be nine hundred years old and now they usually die before one hundred.