In order to access the Temple you first need to go through the Oubliette. To reach the Oubliette, on the first floor you will find a room with a fireplace and a water barrel (water barrels can sometimes be found in other rooms in case you destroy it), after killing any enemies in the room interact with the water barrel and you can push it into the fireplace, go inside the fireplace and press A, this will unlock a secret room on the map with a grate requiring 2 keys to unlock, once you've unlocked the grate drop down and you'll reach the Oubliette.
Note - Before going down the grate I highly recommend killing the boss on the first floor and finding any other useful items, the enemies in the Oubliette are much harder than the first and second floors!
Once you're in the Oubliette continue on like a regular floor until you find a room with a red crest icon on the map, ignore this for now, finish clearing the Oubliette first then come back. Once you've beaten the boss go back to the crest room and pick up the 'old crest' then continue to the next floor through the elevator, you will now need to carry this crest through the second chamber without getting hit until you reach a room with an altar, place the crest on the altar and a staircase will be revealed next to you. If you plan on making it a good run I recommend clearing the floor again before going down but for the achievements sake you can just go down and it will pop straight away!
If you do get hit while holding the old crest it will be destroyed and you won't be able to place it on the altar, but if you get hit you can quit and reload the game then click continue to start from the beginning of the second chamber again.