I do not dispute the merits of Nietzsche's arguments against what he observed to be Christianity. On the contrary I affirm them. The legalistic concept of Christianity and God Nietzsche knew needs to be "grown out of" because it no longer suits the current stage of man's evolving consciousness.
As Nietzsche noted, that sort of Christianity -- a Christianity that is inseparable from mundane middle class values replete with all of its hypocrisies, lies, and resentments -- needed to be overcome. Unfortunately, Nietzsche thought it impossible to move beyond this conception of Christianity. Consequently, he discarded Christianity altogether.
Having said that, I also affirm Nietzsche's search for positive values beyond the legalistic, mainstream, external conceptualization of Christianity. An upsurge of creative being that inspires man toward life and higher values is desperately needed.
Where I differ from Nietzsche is Christ. Nietzsche believed all of this was only possible without Christ. I, on the other hand, believe it is impossible without Christ.
Nietzsche's diagnosis is correct, and the general thrust of his potential cure points in the right direction, but his philosophy ultimately fails because it removes Christ from the equation.
Dostoevsky, Nietzsche's contemporary, also criticized the legalistic, external forms of Christianity, but unlike Nietzsche, he did not fall into the error of rejecting Christianity outright.
Dostoevsky understood why Christ is essential (and he also understood why it is so difficult for people to follow Christ). The Legend of the Grand Inquisitor chapter in the Brothers Karamazov lays it out well.
In the passage below, the Grand Inquisitor rebukes Christ for burdening man with freedom and the duty of value creation (bold added):
"Instead of taking mastery of people's freedom, you augmented it and saddled the spiritual kingdom of man with it for ever. You desired that man's love should be free, that he should follow you freely, enticed and captivated by you.
Henceforth, in place of the old, firm law, man was himself to decide with a free heart what is good and what is evil, with only your image before him to guide him—but surely you never dreamed that he would at last reject and call into question even your image and your truth were he to be oppressed by so terrible a burden as freedom of choice?
They will exclaim at last that the truth is not in you, for it would have been impossible to leave them in more confusion and torment than you did when you left them so many worries and unsolvable problems."
Note added: Nietzsche admired and respected the historical person of Jesus and praises him as an example of "master morality" -- of a person who was able to create and live by their own values -- but he ultimately could not accept the truth Jesus offered because he could not separate this truth from what he defined as the Christ abstraction -- the implementation of slavish values and slave morality in this world for the sake of redemption and the promise of life everlasting life in the next.