Here's a good rule for cooking. It should be in every basic cookbook, and taught in first year cooking-theory classes:
Do not drizzle melted Velveeta over sushi.
No chef needs a rule book to know that melted Velveeta is not a proper sauce for Sushi, and every chef knows how to make a decent Filet Mignon. Bad teachers and amateurs talk about creative chefs as if they "know the rules and know how to break them." That's ridiculous. There are no rules. The great chefs know how food works, and how people work, and how food and people interact with each other. They know that certain foods feel right at certain times of the day, or certain times of the year. They know what they are trying to do with their food. They understand what kind of restaurant they work in. They know what people are expecting, and they know that sometimes people expect too little.
You can't learn this in a book (though a book isn't a bad place to start) and you can't just copy what some other person has done (though that is not a bad place to start, either). It only come from spending time working with food, experiencing it from both sides of the counter, seeking out the best food possible, studying the science of food, studying successful chefs.
Grammar. Music composition. Fashion. Visual arts.
We have been trained in our schools that these fields are governed by strict rules, which we must learn. Only the masters, who really understand the rules, can "break them."
The rules exist for purely pedagogical reasons- to try to distill the essence of what has already come to be known as good style among those people who are steeped in the tradition and understand the genre. Palestrina didn't study Fux. Homer didn't consult Aristotle.
There can be no final rulebook on what is and is not "good sacred music" or "appropriate liturgical music." Some things obviously are, and some things obviously aren't. But there can be no catalog of prudish precepts: thou shalt not syncopate, thou shalt not exceed 117 beats per minute, thou shalt eschew secondary dominants.
To look for such rules - "How can I know that such-and-such is appropriate?" - is to abdicate responsibility for the exercising of personal judgement, and (more) to abdicate the responsibility that each of us as church musicians has to conform our minds and our tastes to the tradition that we have received.