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. Click on the basement door behind the little building directly east of the Clothspinners tailor shop at loc ( 329, -22, -190 ) /waypoint 329, -22, -190 and zone into the instance Spreading the Truth.
When you first zone in there will be several wandering attendees who wander around and see through invisible. (They seem to be maximum level 70)
You can not kill more than 2 to 3 attendees or the quest will not update.
First make it into the door to the right and steal the key from the Celestial Watch ecclesiastic in the room. You can right click on him and do this without killing him. He is not aggressive.
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.
Right click on the locked door and choose "unlock".
Replace the pages on the podium and get back out.
If you have a few wandering attendees attacking you, just keep clicking on the book until you get the update. They seem to do really low damage.