Instructions
Climb up to the upper level of the church to start solving the stained glass heart puzzle.
Open Drake's diary and go through it until you find the page with a heart and two keys drawn on it. The picture is a clue to help you get to the stained glass heart. If you look up at the far side of the church, there is a stained glass heart in the window.
Search for the giant keys on the walls in the church. A large key will be hanging on either side of the stained glass window on the upper level of the church. Pull the keys down to open the door. The stained glass window will slide down and reveal a hidden chamber.
Look at the stained glass windows in the church. There will be another symbol on one of the windows with a heart and two keys. Walk to the window to prompt a cut-scene to start. The window will open.
Step through the open window and into the secret chamber.
Go right after you walk through the chamber and all the way down the hallway to its end to find the treasure vault and get the Jeweled Golden Cross.