The level starts off with the player surprisingly close to a Phfor high above the canyon floor. Pop him with your weapon of choice (if you don't have a problem shooting him/it in the back) and watch the corpse drop down into the canyon. By now you will notice that you're on a ledge that has three doors and one terminal. If you're scared of heights, get use to it, because you'll return to this ledge two more times.
After reading Cosimo's writings, run off the ledge and grab as many shotgun shells as possible and head for the ledge on the other side. It's raining Troopers from the sky!! Ignore them for now and go into the cave after you take out the Phfor that opens the adjoining door.
Work your way around the cave and enter the murky waters when possible. You will need to go towards the cave opening to hit a switch that will open a under water door at the other end of the cavern.
Ride the star platform up and once again you'll find yourself on the upper ledge. If a powerup is need, there is a 1x on a small ledge down to the right.
Note: Due to my carelessness I made it possible for the switch at this point able to halt the door closed or partially open. Take care at hitting this switch (punching it is preferred).
This time leap to the canyon floor and take out the Troopers that fell from the sky earlier. Take the stairs back to the top but this time the third door (the one by the terminal) will be open and this is the exit from the canyon area.
Hit switch on back of ruin pillar to open hidden door.
At the podium you have one chance at getting the 2x on the left ledge. Hit the switch, climb to the highest step and leap for the ledge.
In the cathedral area you will find the main door locked and in this case this door must be opened by a key (the key to salvation??). Dodge the flying dirt clods (or as James calls them 'flying poop'), and head up the stairs and out of this area. Btw, the dirt clod machines can't be destroyed.
The search for the key begins. The locked door of the next structure can be opened by shooting a switch that can be seen through a little door behind the building. Enter the building, explore the halls, and enter the courtyard eventually you'll reach what looks like a small study. Read the journal which mentions the key to salvation from the 'hands of heaven'. To exit the study, jump onto the boxes by the far window, which in turn will lower the window sill. Once outside you will notice the key floating down from the sky (from the hands of heaven) and coming to rest on a small pedestal. This btw, was a major Forge trick and it is not done the way most may think : )
With the key in hand, head back to the cathedral.
Once inside, the first room is a library. There is a hidden door at the end of the far stack which will open when facing it. Leap across the floor stacks for the wall stack and a switch will appear. This switch will open another hidden door. In the Dinning room, exit by running through the fire. The 2x on the table can be reached by jumping down from the upper ledge or running from the draped alcove.
At the base of the cross shoot the switch above the statues head. Enter either arm of the cross to activate the door on the opposite arm. Find the teleporter to exit the map. Salvation baby!!