There's plenty of questions here on multiplayer and questing, but I can't seem to find any answering this particular question.
I just joined my brothers game to give him some items, and afterwards I have to kill Belial again.
When resuming my own game, I was back at an earlier point in my quests, so I left the game, and selected the item to pick my quest start, and I could not pick a point after killing Belial.
Is there some definite information how quest progress is influenced due to multiplayer?
For instance, how can I know, say after killing Belial, that I'm at a particular point where it would be safe for me to join a player that hasn't progressed as far, without me loosing any progress myself?