If you completed the Infiltration Opportunity then you will have a very easy time getting out. Otherwise you need to circle back around to the underground tunnel you took in and use it for your escape as well. There is also sneaking out through the heavily guarded front, but that is not recommended.
Head to the Northeast and go over to the nearby lift. Run at it for a quick ascension to higher levels of Notre-dame. This puts Arno right next to a window he can open with the keys that he stole. Just interact with the lock in the stained glass window and you can get outside very easily. Free-run outside and get away from the church to complete the memory and most of the sequence. Time for a new threat to present itself.