The problem with "Act on what you feel in your heart" is that it's too generalizable. It proves too much, because of course someone else might feel something different and some of those things might be horrible. But if my epistemology is an appeal to an external source (which I guess in this context would be a religious book but I'm going to use "believe whatever Rameses II believed" because I think that's funnier), then that doesn't necessarily have the same problem.

'Act on an external standard' is just as generalizable - because you can choose just about anything as your standard. You might choose to consistently act like Gandhi, or like Hitler, or like Zeus, or like a certain book suggests, or like my cat Peter who enjoys killing things and scratching cardboard boxes. If the only thing I know about you is that you consistently behave like someone else, but I don't know like whom, then I can't actually predict your behavior at all.

The more important question is: if you act on what you feel in your heart, what determines or changes what is in your heart? And if you act on an external standard, what makes you choose or change your standard?