Nietzsche is highly critical of Christianity, especially Christian ethics. He sees the Christian religion as being based on an ethic of pity and self-abasement, an ethic that speaks to the weak and sick in spirit while disparaging the strong.
Yet despite these characteristics, Christian morality as it has developed in the West has achieved remarkable power, overthrowing the previous system of ethics that emphasized all that was strong and assertive, the kind of ethics endorsed and practiced by the ancient Greeks.
Due to the spread of Christianity, what was once a religion for slaves and apostate Jews has become part of the political establishment right throughout the West. That doesn't mean for Nietzsche that the fundamental essence of Christianity has changed; according to him, it's still a religion that valorizes weakness and submission. But because of its alliance with secular power, Christianity has achieved the kind of dominance of which the Romans could only have dreamed.
In the modern world, Christianity has had what Nietzsche sees as a baleful influence on politics. The demeaning pity ethic of Christianity has become secularized in the form of socialism, which Nietzsche attacks for being an elevation of the weak in spirit at the expense of the strong. A thoroughgoing elitist, Nietzsche is profoundly contemptuous of the idea of social leveling, of reducing the strong to the level of the weak, which he sees as being at the core of the socialist program.
For good measure, he is utterly scathing of democracy, seeing it as the triumph of the mediocre and the second-rate. All this, in Nietzsche's eyes, ultimately stems from Christian morality with its resentment for society's stronger specimens.