I am playing thru the campaign, and am not sure if I am collecting all of the shrines.
1 - Is it essential to collect each and every shrine?
2 - Are shrines the only way to increase devotion?
You can increase Devotion points in Crucible, there are around 70 Shrines in all difficulties (maybe even more, don’t remember exactly). You can have maximum 55 devotions, so don’t worry.
I’d recommend to explore all the map with your two first characters to know how the map looks and where stuff is. Therefore you’ll not have to worry about missing them. I now “speedrun” normal difficulties and I still get at least 27 out of 28 shrines in vanilla region (sometimes forget one shrine in a side cave).
You don’t have to collect Shrines because you only get items off them if you have collected all 55 devo points.
Firstborn, there are more shrines than devotion points available so you can skip some if you want. All are available in Normal so as USER_NAME_01 said probably best to explore everywhere and find them since it’ll give you a good boost with all the devotion points you’ll get. Fewer are available in Elite and Ultimate. The GD Wiki has a list:
https://grimdawn.gamepedia.com/Shrines
Once you have the 55 points maximum the shrines will only give you loot.