a rogue (me)

This is what's called a "DMPC". Normally, this is a very bad idea because most DMs that do this will have their PC take the spotlight off the actual players of the game. It looks to me like you will be stealing the spotlight because you've made a rogue, so all the sneaking around, trap finding, trap disarming, back-stabbing, and unlocking things is going to be your character's job. If you must have a character in the game, make sure that the players are the ones in the spotlight, no matter what class you are playing. So let it be them that tells your rogue to go hide over there, backstab that thing, unlock this thing, etc. Don't use your PC to do many things for the party until they come up with the idea to do it. The overall story is for you; the adventure is for them.

The cultist will have had a key on him, and that will dispel an illusion that was there, letting them head further into the temple.

Typically, a "key" wouldn't have anything to do with illusions. You might want to make this a key that opens a secret door, leading further into the temple.

friendly Kobolds?

Since the PCs are already going to be facing goblins, you could have the goblin tribe split into two factions. One group wants to help the cultists because they think they're actually going to get some big reward for it, the other group thinks or knows that their tribe is only going to be food for the evil god, so they want to help the PCs.