Owning a Great Temple(GT) in a Kingdom(KD) usually signifies that you are the owner of said KD. You can occupy more than one GT with different troops size everytime you took a GT.
Here's a few tips if you want to occupy your first GT:
1st: Make sure you scouted your target(GT) first, going in blind is just dumb.
2nd: always make sure that you have a Dominion (Dom) in a KD of the GT that you want to occupy because the farther it is for your troops to reach your destination (you attack from a different KD) means that the troops inside your target(GT) would increase exponentially.
3rd: bring at least about 40,000 of mixed troops/units(or 3,000 of each troops/units) but focus mostly on Royal Guards (RG) because they will took the brunt of the enemies attack. You can bring more than 40,000 troops but it'll be an overkill and a waste of food resources.
A GT can't produce resources, but you can collect taxes from every trade that happens in the KD where your GT belongs and buy resources from the tax office building, there's a temple of which you can hire heroes from, a Level 20 Barracks and Level 20 Arsenal, a building called Sanctuary that can produces special unit called Angel.
Taking a 2nd GT will be harder(brought all of your troops) and you must expect somewhere around 10,000 to 15,000 troops casualties.
Note: while the Angel unit is powerful they are really expensive to produce.
2nd Note: taking over someone's GT will surely pissed off the owner of said GT and will lead to conflict, if you're in a guild it will escalate into a war.
3rd Note: you can set the taxes of a KD that you own from the GT, it ranges from 20% to the max of 40% tax.
4th Note: setting the tax in your GT by 40% is considered a tyranny, and will leads to other Lords(Players) taking over your GT.