The forgotten chapel can be reahced from the Inner Courtyard.
The initial section is mostly filled with weak spiders and flying imps. When you reach the altar, there's a few steps on the wall behind, which are used to reach a push wall. This moves the altar to reveal a secret pit.
When going down the pit, you should be first stopped by some webbing. This first room is connected to a small dark room where you find a body encased in webs. The tailor's key should be here - if not, hack the body for the key.
Break the webs and continue down to the tunnels below. Only a few enemies (a golem and two archers) are in the tunnel and are scattered. The end of the tunnel leads to a teleporter that brings you back into the chapel.
The teleporter drops you above a pit, and there's archers throughout the room. The exit is barred, but should be open if you moved forward after going through the teleporter, which will land you on a button. If not, you