There is no reason that all party members must be sourcerers.
I am sure there are many people in this world who do not like the divine order and/or have a very good reason to get away from them. Some people would surely think that it is better to join sourcerers than to fight/escape the divine order alone.
Just because somebody is not a sourcerer does not mean he is weak. (Many enemies, including bosses will be non sourcerers I guess.) It looks like your source powers depend also on your background and race. So a non sourcerer companion could do some things you cannot.
about obsidian: I played KotoR2 recently. The main char and several companions are force users and that is importent for the story. But you have also non force users in your party (people and droids) who can do some things you can not.
some random ideas for D:OS2 non sourcerer companions:
- A barbarien who has some special enrage abilities. He slaughtered many members of the divine order (there can be tons of reasons for that) and now he wants to get away.
- A cursed mage. She is able to see/hear things other cannot. Like visions from other places/worlds or see the past/future. Somebody has cursed her (or her ancestors) and her gift is driving her insane. She is not a sourcerer (I have no idea what the source is at all) but I am sure some fanatics do not like her as well.
- A fanatic heretic who hates the divine order. He is absolutely convinced that the gods do not exist and the divine order spreads only lies. Maybe he is a satanist (gains power from demons) or a nihilist (gains power from the void).
There can be tons of reasons why somebody joins sourcerers, even if sourcerers are hunted by the divine order.
PS: The cursed mage often has one vision that drives her crazy. Somebody in another dimension is staring at a flat object and pushes buttons on an alien machine and this way he controls her entire life. Her vision tells her to pull the plug to free herself, but she does not know what it means or how to do it
Prof. Dr. Dr. Mad S. Tist 
