So today the DM gave the party a Bag of Holding.
Me and the other major criminal of the party wondered if we could make a bomb out of it, going over ideas like filling it with air, fire, lightning, even Magic Missiles.

Until we realized the perfect answer: Glyph of Warding.

A Bag of Holding only has a weight limit, not a volume limit, and it wouldn’t even matter if the DM put a volume limit on as well, because Glyphs of Warding are both weightless and two-dimensional. So, you can fill a Bag of Holding with infinite Glyphs of Warding.
Next, a Bag of Holding appears to be a bag, but is actually a gateway to a pocket dimension. Because the only thing moving when a Bag of Holding is moved is the outer half of the gateway, nothing inside the pocket dimension itself is actually moving, therefore meaning any Glyphs of Warding inside won’t be triggered by movement.
Now, stepping away from the Bag of Holding and Glyphs of Warding for a moment. When you cast any spell that specifically manipulates matter, such as Mold Earth or Shape Water, more than once on the same area at the same time, the very molecules in that area will start to smash into each other, either fusing together, reacting, or splitting apart. As the molecules split apart, some of the individual atoms will begin to slam into each other and fuse into a larger atom, or in other words, start a fusion reaction. At the same time, other atoms will split apart from being manipulated in different ways multiple times over, which creates a fission reaction, better known as an atomic explosion. As the fission and fusion reaction happen in tandem, larger, less stable atoms formed by fusion will start to split in the fission reaction, making the explosion grow. In the meantime, surrounding atoms and molecules will do the same from the sheer energy being released. Eventually, this f*cking destroys a significant chunk of the planet, enough to make the planet no longer spherical enough, and because gravity and physics, the planet will warp to try to return to a spherical shape, causing more destruction. The moon(s) will also drift away from the planet due to its loss of mass, or if the planet’s lost enough mass, it may end up like Pluto, orbiting around a point not in the center of itself. The planet’s orbit around the Sun would also be f*cked up, and that would kill off just about anything that some how survived everything that came before. If it’s really bad, the planet may slam into another celestial body and be entirely obliterated.

And back to the Bag and the Glyphs. If you set each Glyph of Warding to a spell like Mold Earth or Shape Water, and just fill the Bag of Holding with those, you can unleash the unholy planet-destroying power described earlier at will.

We dubbed this idea “The Bomb”.

The DM outright said, and I quote, “I think I’m gonna have to put a stop to your Bag of Holding shenanigans.

So, yeah, just putting that out there.

Who tf said you need to be an artificer to make a bomb out of a Bag of Holding?

Under the rules, this does not work. Mold Earth specifies that you move the earth “across” other earth - you cannot move it into space other earth already occupies.  Shape Water also does not allow you to do anything that would compress the water in this manner. 

You also did not read the bag of holding rules as there is a clearly stated volume limit.  This limit is 64 cubic feet.  That is plenty of room for the materials to all just slide around past one another, as they would under the rules for both spells you listed.  For example, 500 pounds of water (the maximum you could put in a bag of holding) occupies less than 8 cubic feet of space. Earth, as something even denser, will occupy even less space.

Accordingly, this falls firmly into the category of “clever” ideas that require the DM to ignore the plain text of the spells and items in order to function.  No DM worth their salt is going to bend the rules to allow this -even the most “rule of cool” DMs would roll their eyes at any player who thinks this is cool.  In fact, as a general rule, nothing in the rules as written really allows the player to destroy the world - that is the purview of the DM. 

Now, while not apocalyptic, it is possible to turn a Bag of Holding into a weapon under the rules - but you would require a different item, such as a Portable Hole.  Whenever a bag of holding enters a portable hole, it creates a momentary rift to the Astral Plane, with no save for creatures within a ten foot radius.

It would not be hard to build a trap taking advantage of this - all you have to do is orchestrate a situation where a bag of holding falls into a portable hole.  For example, a pitfall over a portable hole could be a fairly effective trap for a DM with a high enough level party that they have a bag of holding and can deal with something that degenerate if a trap. As a player or DM, you could also bait the pitfall with a bag of holding, so it also falls in when the pitfall is triggered.  I’ve always thought this would be a pretty fun way to start a campaign - the party triggers such a trap and get whisks away on a magical adventure.