The characters inside will see through invis but you can get pretty close to them before they will react. Try not to kill too many of them. If you stick to the corners and are patient they will keep wandering around and you can make it through only having to kill 1 or 2.
Head to North Qeynos. The Guards are level 55 at the gates, Lower level characters can avoid the guards by go in through the Peat bog to the Vermin's Snye to Down Below, then North Qeynos. Go to basement of the little building directly south of the Clothspinners (tailor) shop ( 329, -22, -190 ) /waypoint 329, -22, -190.
First make it into the door to the right and steal the key from the guy in the room. You can right click on him and do this without killing him.(If you kill too many people in this instance you will not be able to replace the sermon. 4 Kills Max. Apparantly, this is now down to 3 kills max counting the ones downstairs. Oddly enough I went over 4 kills counting the upstairs ones and the quest worked.)
Then make your way up the stairs. There are only 2 up here, but they are standing on either side of the podium you need to get to, so you will have to kill them. (If you inch up the middle slowly, you can place the sermon without aggro, just keep right clicking until it lights up.)
The guards aggro'd me the moment I stepped through the door. 8/27/10