I was reading a previous thread about turn undead revealing what god a cleric worships. The reply says:

The Beggar

When a cleric uses the turn undead power he/she holds their holy symbol they should always have on person out prominently infront of the undead in question and commands them in the name of the diety served to be destroyed, be gone, be under his power, etc.

and:

Of course, as a cleric (with very, very, very few notable exceptions) everyone should already know who you serve. If they don't, you aren't being a very good cleric in spreading the teachings of your God about.

I was wondering if a Cleric of a god with the trickery domain (like Gargauth or Mask) would be one of the exceptions?

Naturally they would only want to reveal themselves at specific times. They might even have their holy symbol displayed somewhere, but never draw attention to it unless needed (like turning undead).

A separate but related question: Does casting a divine spell also reveal your patron?

You are praying to recieve and cast your divine spells, so yes, casting divine spells would reveal who you serve.

The so called "secret" Gods (for example, Shar, Mask, Cyric) have clergy that may not openly preach for fear of being killed outright. That still does not remove the responsability of the priest to 1) preach, and 2) convert, and there are many ways to do this while not preaching on a street corner. The deities demand that their clergy act as such.

As to divine command of undead (or turning), you are commanding the undead in the name of your deity to obey you as a divine representative. Absolutely you would use the deities name to do so. Let me give you an example of a cleric of a "secret" God promoting his faith the right way.

Cleric of Mask: A young rogue turned holy man of Mask through connections with the churc he had made over a long time within a previous theives guild, the enterprising priest sets out on his own for a new town recently set up along a river. Setting up shop as (insert something benign here), he secretely and slowly recruits rogues and thugs from out of the town to come in to aid him in setting up a smuggling ring, and in controling much of the growth of the city through nefarious means. Late at night, he gathers the crooks together to hold worship services to Mask, which he leads, each man there inducted into the secret brotherhood of (insert cool name here) with the penalty of death set to each if they should betray. Every new thug/crook that comes into the town, is then recruited and worships...and if not, they die.

Cleric of Mask (not doing what he should): This young cleric of Mask was a young thug who just woke up one day and decided to be a cleric of mask, because mask gets the trickery domain, and is that is cool. Desiring uber divine wands of invis and the chance for imp invis, he sets out adventuring with parties, never invoking the name of Mask, let he be found out as a cleric of Mask. While wearing full plate, he runs around into dark corners just ahead of the party to pray and cast his spells so they won't catch on. Then, when he has achieved the spell sets he wants, he attempts something mildly evil--like threaten someone. But that's alright, because he is a cool Cleric of Mask. (Who gets spellfailure, for not playing a cleric)

You are praying to recieve and cast your divine spells, so yes, casting divine spells would reveal who you serve.

I do not agree with this myself, so hold off on considering this official policy until the DM team has a chance to discuss the matter!

My own view is that most clerics will natter on incessantly about their deity, and this should be reflected in their RP, but that simply by mechanically casting a spell nothing is revealed.

Correct, and I should clarify. In Faith and Pantheons as well as PHB it states that

Clerics and priests pray to recieve their spells

My post came across as answering the question wrongly as I re-read it this morning. We'll have to come to a concensus as a DM team and will give you an answer like Howland stated.

Quote: You are praying to recieve and cast your divine spells, so yes, casting divine spells would reveal who you serve.

I do not agree with this myself, so hold off on considering this official policy until the DM team has a chance to discuss the matter!

Has a consensus been reached on this point?

I am of mind that in casting the spell you do not necessarily need to name your deity. You receive your spells via your day's prayer, which is likely where you'd invoke them.

As a priest, you should generally not masquerade as a cleric of another faith either, unless you are a priest whose patron is someone like Cyric or Shar, deities dedicated to secrets and deceit. The Gods would generally not approve of someone imparting their blessings, miracles and gifts in the name of another deity, furthering their cause in part.

If you are a priest of one of the aforementioned gods, please do not pretend to be a priest of another deity at all!

We have decided that casting a spell mechanically does not reveal patron deity, however most clerics really should constantly be making clear what patron it is that they worship. Using the turn dead ability _does_, however, reveal deity as canonically it involves the showing of a holy symbol.

(Of course, not at all PCs will recognize holy symbols).