@kirakrakra: Το σύμβολο της πίστεως is a special case, since it is a set phrase used by the Orthodox Church. Το σύμβολο της πίστης—though grammatically correct and maybe sometimes used by more modern(izing) clergy—is somewhat unusual and might offend some more traditionally minded people's sensitivities.
Apart from that, for all feminine nouns ending in -η in the nominative singular the genitive singular ending in -ης is the more useful one to learn, since it applies to all nouns of this kind, whereas the genitive singular ending in -εως can be used only if this noun ends in -εις in the nominative plural. So η δύναμη (pl. οι δυνάμεις) forms the the genitive singular as both της δύναμης and της δυνάμεως, whereas η γραμμή (pl. οι γραμμές) forms the genitive singular only as της γραμμής. So the -ης ending has you covered in both cases. With the genitive ending in -εως there is also the risk for a foreigner of sounding a little bit stilted. Having said that, I think it is good to know that genitive ending in -εως exists, if you happen to come across it.
In the case of The symbol of faith my view would be to use Το σύμβολο της πίστεως as the main answer, and Το σύμβολο της πίστης as an optional one (whenever the sentences get unlocked), and have everyone happy: The Orthodox Christian will find his answer accepted as the main one, and anyone using the grammatically equally correct -ης ending will pass as well, just with a reminder that there also exists another variant with -εως. This is just an opinion though.
My "faith" is to avoid linking words to religious doctrines. Orthodox Christians are trying to be considered owners of the literary Greek language and I do not want to participate. I think we should accept the words for their true meaning and not for religious.
Your friend Kleanthes