As a particular example, consider how Nietzsche, in Thus Spoke Zarathustra, notices that “up to now [revenge] has been mankind’s chief concern”. (Penguin 1961: 162) Nietzsche’s Zarathustra counsels against this spirit of revenge — and in it’s place a mental state of affirmation.
Yet, herein we have a problem: there are those who would misread an injunction against ‘revenge’ as an injunction to remain passive, simply because Christianity has taught them over the ages to equate inaction (allowing the Lord to avenge one) with ‘morality’ — and, also because religiously inspired action (that is mistaken for all kinds of action) is inevitably reactive rather than creative.
Christianity has so dominated the general culture’s understanding of what it means to act (courageously, for instance), that it even gives Nietzsche’s writings a false flavour of moral prohibition: Instead of “thou shalt act to morally justify oneself as being good Christians”, the postmodernist who has internalized Christian values reads the Nietzschean injunction as: “thou shalt prohibit thyself from acting, since any acting only takes place on a moral basis — and you ought to be above that.”
So that is the danger of mixing Nietzsche and Christianity. It leads to a feeling that one is prohibited — by morality, as it were — to act. Nietzsche, however, holds that a noble person acts nobly, without any need for a specifically moral framework to give meaning or direction to one’s actions.