I'm running Princes of the Apocalypse for 3 players and they diplomacied their way in to the Feathergale Spire (front-face of Air Cult, they don't know that). So far, they've gotten really interested in their false goals of protecting Waterdeep and the surrounding area of evils.

Well, they successfully defeated the Manticore and impressed the Lord Commander. One of the initiates put in a recommendation for them to join the as a Feathergale Knight. They did okay with talking to the Lord Commander but do want to join. I'm sending them off after the Reaver Ambush (Water Cult ambush) to keep the Reavers from taking the caravan's goods. My thought it so have one of the actual Knights follow from the sky in secret to see how they do on this initiation quest to be let in. When the characters beat the reavers (assuming they do), the Knight swoops in and tells them the caravan is a theives' guild - but is actually a merchant caravan of food stuffs - and by taking it will be a great victory.

Upon succeeding, I'm going to offer them a creature egg of a flying mount as part of the induction to the Feathergale Knights. They must raise their own mount so it bonds with them (and for the nice incentive of why to stay with the cult). And perhaps ultimately, if they stay long enough, I may have them protect the Elemental Node of Air from some vile ravagers (adventurers) who seek to come and loot the group and take the powerful artifacts for some evil purposes (save the land).

Beyond this, I'm not sure what quests I could send them on on behalf of the cult and looking for ideas.