Hey all!

Been struggling on this idea I had, and figured I'd put it out there and see what the community can offer...

THE CONTEXT:
I've got 4 lvl 3 players, and they are taking someone to a shrine that, upon the right ritual, can heal an NPC of madness (something the party is not currently equipped to do). While some of/all of the players are performing the ritual, they are attacked by X monster (spiders, wolves, haven't decided, not too worried about this piece). My goal is to create an encounter where players/the party have to decide who will focus on the ritual, and who will defend themselves from the threat, until the ritual is complete. 

THE PIECES:
I don't have a full fledged plan for this 'ritual', but here are the piece I'm trying to work in:
Religion skill use: I'd love to see players use this skill to "pray" for this ritual. Thinking of setting a high DC (50?) so there is some value to dedicating multiple player attention to the ritual vs. the combat. My concern? A non cleric player has a higher modifier than our cleric, and I'd love for this to be an opportunity for our cleric to shine. 
Concentration: I'd like to have players that ARE focusing on the ritual, need to maintain concentration during their turn (action only?) to be able to make the skill check. My hope here, is that this also adds value to focusing on the combat.

THE QUESTIONS:
How do I build this in a way that allows our cleric to shine, and acknowledges their connection to the god this ritual speaks to, despite their low religion skill modifier? 
Does this ritual require concentration for a players whole turn, or is that too much of a cost? Should it just be an action? 
Anything you think would be fun to toss at them to fight while the ritual happens?
Your questions, any other thoughts, or feedback?


Thanks for anything!

The cleric should have either a bonus added to their rolls or advantage since it is their god.

Is it a cumulative difficulty class? Do the players add all of their attempts together? I think that would be good idea, and I think that maintaining concentration is good for between these attempts, so that is not too easy. You can have one or more players devoting their actions to making and maintaining these checks, and the other players protecting them so they don’t have to start over.

The obvious way to allow the cleric to shine is to either not use skill checks, or float the checks to Wisdom. However, I would caution against this adventure design, because it's boring for the person who's stuck doing the ritual.

One variant you might do is have the madness which is being cured be a result of possession by a ghost, in which case Turn Undead is the key power (or perhaps you're doing a ritual exorcism).

As a Cleric player (often), I find that opportunities to interact with rituals and ceremonies are hard to come by (our DM is more against having divine interaction than for, which is strongly rooted in his personal agnostic views).  I would prepare a series of questions that you want the cleric to answer as a supplement to performing the ritual.  At the beginning something like, "What do you ask of your deity in this ritual?" so they have spell out what they are trying to do.  Describe some hand motions and incense burning or whatever it is you want to flavor the ritual.  Ask them to lay their hands on the person afflicted, "What thought do you project as you lay your hands on Jane Doe?"  This gives them a chance to think a single word focus for their ritual that you can use to emphasize what is happening. 
Let the combat or whatever difficulty it is begin.  Roll for concentration.
Let a round or two go by and ask, "What focus or object are you using to maintain your concentration as you project _____ in curing this person?"  If they have a good answer (rubbing their emblem with their thumb, saying the focus word over and over with an incantation, etc) then give them advantage on the concentration check.
Continue along with the ritual for however many rounds you want this progress (or until a threshold is reached).  Don't ask a question every round, but every two or three rounds have something that progresses or hampers the ritual and ask a question they have to answer or that they have to move to a different part of the room (light the candle or the brazier... throw salt in a ring around the person... turn and face the sun, etc) and describe more of what is happening or intonations or how the smoke ascends, etc.  Either way, finish it with strong description that involves all the parties...

Basically you are giving them an opportunity to have buy in to the ritual in a mental level so that they are engaged with what is going on.  They don't know how long it will take and they don't know if they are giving the "right" answers or not (to be honest, the answers don't really matter, but if they give thoughtful answers give them advantage on the concentration or the progression check).  Just think about how you want the ritual to proceed with some good description and opportunities for the players to interact and respond other than just make another dice roll.  If those happen, it will be memorable and they'll enjoy doing rituals or other ceremonies in the future.

Just got home from work and reading through. Really appreciate the input ya'll!
I'll have to soak it in and let ya'll know what ends up happening. 

Thanks!

You could allow the ritual to succeed when the players hit a cumulative score, like they are doing damage to it — effectively the ritual becomes another enemy. The ritual has x hp, the characters spend an action concentrating on it, they can do a religion check worth of “damage” to it. Allow the cleric to do it as a bonus action and/or an action, then they’ll be the star, since they can go twice a round. It gives everyone a chance to dip in and out, and provides some interesting tactical decisions about if it’s better to “hit” the ritual again, or one of the attacking enemies.

We have allowed a cleric or paladin to expend Channel Divinity in certain situations for a benefit where there is a connection with their specific divine host.

One way to handle multiple checks required is similar to what Xalthu outlined above - you treat the DC like you would HP (and set it much higher than usual), and each check subtracts that value until it hits 0. CD could be a one-time large shot to that pool, or it could allow BA checks or something.

You may already have this, but you need a good reason why they can't just stop the ritual to focus completely on the fight and then pick it back up or restart when they're done. Cause that's the first thing a party would want to do. If you're worried about a certain character out-ritualing the cleric, make sure you're really harrying them with monsters to draw them into the fight.