I think you are the
We are the ancients, though we still sit among you.
Plate tectonics probably started on Earth over 3 billion years ago, currently there are seven recognised plates.
We have many forms, and even more layers
Wikipedia gives an extensive list of plates. Processes like subduction mean the plates occupy several layers of the lithosphere.
No strangers to pressure, we use it to shine through
The plates often move and cause areas of pressure in the Earth's crust. This can be the cause of diamond formation.
We'll hold you up while you kneel for your prayers
We live on top of tectonic plates.
We have patience beyond compare
The plates are very slow moving in general, typically from 0 to 100mm per year.
We'll crack and break and never whine
But when we fight, you'll know we're there
A clashing of plates at a convergent boundary can cause earthquakes.
Especially if you toe the line
Toeing the line means being near a plate boundary.