A slightly annoying glitch that happened to me...

...a certain priest of Mara seemingly got stuck in a set of stairs and refused to follow me up to the door he needed to unlock in order to continue the quest.

Solution:
I beat the c**** out of him, causing him to stagger up the stairs and finally go down near the door...as he was an essential NPC he eventually recovered and proceeded to open the door and continue our mission, without any hard feelings or anything. See essential NPC's can be useful...

Yes I've had that too. I've shunted him up the stairs a bit with the moveto player console command and he seems to move again. Using unrelenting force to blast him up the stairs didn't work. Only had it happen since the last patch.

That quest worked fine for me the first time around.

But I tried it recently and have that same problem you had. I ended up using Unrelenting Force to toss him up the stairs. When he was near the door he unlocked it and the quest was able to continue. His AI seems to have got bugged after the latest patch.