On this page of the Horizon Forbidden West guide, you will learn how to get the A Church? Really? ornament.
First, smash all the obstacles in the building using Pullcaster and Igniter. This will clear the way for the crate.
The crate is located on a platform. Pull it down using the Pullcaster and then move it to the wall with the door.
This will get you to the roof, where you'll find one more crate. Move it along the only path allowed by the game. It will end up in a small nook. Line it up so that it fits perfectly. Then take the second crate and place it on top of the previous crate.
Move the structure so that it's positioned next to the tower. This way, you will be able to grab onto a stable ledge and climb to get the key.
After that, move the crates towards the wall with the door again. Now you can climb up and open the door using the key found.
In the middle of the room you will find a hook for the Pullcaster. Use it and make a hole in the wall.
Use the hole to pull up the top crate and move it towards the tower wall. This way, you will reach a handhold that you can use to climb up to the room with the Ornament.