Muttonchop. That should be the answer to any sending cantrip sent to a deity. Just that one word, Muttonchop. let the cleric take it for what they feel it's worth.

Or just tell the player the Sending went through but there is no response. Or, if you need to relay some foreshadowing information, then do that. But be careful or they will be casting Sending every turn they get.

Quote from ThriKreenWarrior >>

Muttonchop. That should be the answer to any sending cantrip sent to a deity. Just that one word, Muttonchop. let the cleric take it for what they feel it's worth.

 

Or just tell the player the Sending went through but there is no response. Or, if you need to relay some foreshadowing information, then do that. But be careful or they will be casting Sending every turn they get.

 Sending is a third level spell, not a cantrip.  It will work just fine on a deity, but why would you bother?  Consider your options:

• The Sending spell: costs you a spell slot of L3 or higher.  Odds of success: 95%.  Message length: 25 words or less.  Deity hears your message, recognizes you as the sender, and can answer you with 25 words or less.  Deity is guaranteed to understand your meaning.
• Perfectly normal prayer: Costs you no spell slot.  Odds of success:  100%.  Message length: Infinite.  Deity hears your message, recognizes you as the sender, and can answer you with as many words as it likes.  Deity is guaranteed to understand your meaning.

There's just no reason to ever use the Sending spell to send your god a memo.  But you can do it, and it will cost you.

You are correct, I was mistaken. I had Message stuck in my head for some reason. 

Quote from TaynOfDarkwood >>

I'm gonna say "yes, but."

The message gets through. Your 25 carefully chosen words arrive in your deity's Spam Folder. They sit there for a while because your deity has stuff to do. You know - it's a deity - kinda busy - can't stop deitying just to read every cantrip email sent by every follower in the multiverse. So yeah, your message gets through. Good for you. But... if you're expecting a response, sure, go ahead and have a seat right over there and feel free to hold your breath if it'll make you happy.

Eventually maybe your deity will have a bit of downtime in between battling demonic hordes and causing/preventing earthquakes, and maybe they'll browse through the 4,883,761 messages on their answering machine. And maybe they'll actually make it to yours before nodding off or getting bored and spreading a plague or something just to break up the monotony of immortality. But since all you sent was a cantrip, you'll be lucky if your message even gets a swift nasal exhalation of a response before being deleted and forgotten. I mean, come on, you sent a cantrip to a deity. That's like giving your wife vacuum cleaner bags as an anniversary present. I mean, technically it qualifies as a "present". It was wrapped, there was a bow, it had her name on it, all the prerequisites were met.

I'm just saying, you're probably not gonna get the response you were hoping for. 

 I think you might be conflating message with sending, no?

It’s your call about when the PC is “familiar with a deity. Do they need to know them as presented in religious text or to have had tea with the deity? It is written this way so the DM can make the call depending on how they want to play it.