Once you're inside, go to the altar and examine it. After that, pick up a statue lying on the right side.
Now you have to help your companion to climb up and wait until she drops down the ladder.
After you go up, get back to the direction of the altar and place the statue on the podium there. Nothing will happen, but you'll see an inscription on the wall. You'll learn out of it that you need another similar statue.
It is hidden inside the chest in a nearby room. In order to find it, you have to go to the exit direction. The room is located on the left.
When you have two statues, place one of them on the podium next to the inscription. Another one should be place on the other side of the altar.
A door leading to the room with two levers will open then.
Pull one of them and then quickly run towards the shooting statue and press a button at its base (A).
Use the same method to deactivate other statues, moving all the time across laser, to not being hit. When you're done, point your weapon at the nearby mirror.
A further way leads up the stairs, where you have to aim your weapon at the mirror placed behind the grate downstairs.
When you reach the next door, turn left and shoot at the mirror on the top.
A bit further prepare for the fight with zombies. Defeat all opponents and shoot at the mirror place on the ceiling on the left.
Behind the next door you'll find a blue statue - take it.
Go to the other side, where you'll see two terraces and bell towers. Your task is to shoot at all five bells. Go to the balcony on the right and shoot at two large bells on the left and a small, rotating one, on the turret.
When you go to the left terrace, you have to shoot at the large bell and a small one.
When you're done, a grate will open, behind which you can find another statue. A bit later also a passage to the lower floor will be unlocked.
Get through it and put statues on podiums on both sides of the altar. In this way you'll open a passage, from which an acid spitting zombie will come out. You have to fight it, shooting also at civilians transforming into beasts.
In order to kill the large creature, use bombs. When the beast falls down on its knees, approach it and kick it, finishing the fight.
Next to the corpse you'll find a key card, which opens the door under the altar.