OK, where exactly are you stuck? Still in the cathedral or already in the dungeons? If you are still in the cathedral, there is a button (hard to find) behind the icon near the door that leads to the stairs upwards. Pushing this button will unlock the locked door behind the altar. Open the door and find the levers there which will open the altar.
If you are already down in the dungeons - come to the big hall with lots of crates. Go up the stairs on the other side of the rail-tracks. Find your way through the crates further on into a quite dark tunnel. There is a light switch on the right side of the wall. Soldiers in steel-armor there. Move to the end of the tunnel where there is a solid locked door. You get a new mission objective there (go back to the big hall and to the right, turn switch on. Return to the area with boxes, on the left is a switch - turn it on. Back to the dark tunnel with the rocket. Pull levers to the left and then to the right - RUN BACK because the rocket will blast the doors.