Personally I can hardly imagine a religion in Stellaris, I mean, agressive, pure religion in the age of hyper advanced tech and contact of dozens of extremely different life forms?
Don't get me wrong, religions have extreme adaptive abilities but I think the form of religion surviving till so advanced era would be some ethereal, watered down spiritual mixture of christianity and buddhism, or some ideological/cultural stuff.
I mean, despite all Creationist backslash, look how great implications Evolutionism and modern science and prosperity had for the religion - and in the developed parts of the world religion is systematically getting more liberal. Now put 10 times bigger scientific revolution and additional 200 years... I think Ethos issues covers religious part in Stellaris.
Any ideology capable of convincing - or being comprehensible for - extremely alien species is going to ne based on princies universalist enough to be the matter of 'ethos', not religious cults.