If some of the condemnable actions or beliefs of past saints are excusable because of the cultural and historical context, is it inconsistent to say morality isn’t determined by one’s society and culture?
There is a difference between objective morality and subjective moral culpability.
Someone can be do something that is objectively always wrong, but because of circumstances may not be morally culpable for that wrong action.
When we speak of people from bygone eras and their actions, we can hold that something was objectively wrong but that the individual may not bear moral culpability or may not bear full culpability.
Catechism of the Catholic Church:
1790 A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed.
1793 If…the ignorance is invincible, or the moral subject is not responsible for his erroneous judgment, the evil committed by the person cannot be imputed to him. It remains no less an evil, a privation, a disorder. One must therefore work to correct the errors of moral conscience.