There are many ways you can handle this. Allowing this all the time is overpowered and will likely get boring, but equally disallowing it all the time is boring and stifles creative play.
You have to find a balance, and that balance depends on the party. Personally, some of the most fun D&D I've had is when we make a half baked plan and manage to convince the bad guys that we are friends or similar, but other people prefer more straight combat encounters and want to get straight to the combat. You have to read the party - if they are all getting really into the strategy, then make more allowances for them to get it to work. Conversely, if 3 people are ready to fight and 1 person is trying to make this work then consider making it a simple intimidation check to make a couple of goblins run away while the rest attack.
That said, it seems that your party is into this sort of thing, so here's a couple of things to consider when coming up with (cool and fun) ways to deal with these situations.
Goblins aren't stupid
While it can be fun to play goblins are really stupid, you don't have to, and RAW they are of average intelligence. Goblins can figure out this isn't a miracle (or at least, not a miracle of their god), and then it's not going to be very effective. Or you can take the best of both worlds - the general goblins don't realise, but the goblin shaman realises what you are doing and convinces the goblins to attack (or starts putting up a second show to convince the goblins to attack).
Do they know enough about goblin culture?
Perhaps the goblins are submissive in the face of their deity. Or it could be that only goblins are 'holy' enough to see their deity, and the party (who are presumably not goblins) will now face fanatical goblins who will do anything to see them dead.
Since you're running a module with experienced players, they are more likely to know about things in the world already, but their characters don't necessarily know what the players know. Do they know who the god of goblins is? What they look like? How they act? You could make them make a religion check to figure out if they know enough and to avoid a faux pas ("I am the god of wrath! you should show mercy to these adventurers" would probably raise a few eyebrows). A deception check may be necessary to keep your lies straight.
There's also the consideration of language. The god of goblins will likely speak goblin - if the party doesn't know goblin, it will be a hard sell to say why the god of goblins is speaking common.
"Yes, but..." or "No, but..."
This is something seen in a few other RPGs, but just because it's not written into D&D doesn't mean you can't use it. Succeeding at a cost or failing with a benefit is a way to reward the creative play without making it overpowered.
They succeed at their religion check, but you can't remember any suitable goblin deity except XXX, a god who demands constant sacrifice. Now the adventurers are faced with a choice - is avoiding this fight worth increasing goblin raids on neighbouring villages to appease their god?
Alternatively, you fail a deception check, but the goblins are distracted by the show long enough that that you can sneak away (or at least get a head start), or sneak into better positions to get the drop on them.