Nietzsche I'm told wasn't interested with the metaphysical truth of christianity being convinced it is obviously untrue and was more interested in its social effects. But what were those reasons in Nietzsches case that made him think it was untrue? Was it a family death or all the suffering in the world that seemed inconsistant with a loving god?
It is not about God, at root. It is not the theology of Christianity that he openly disrespected. It was the mechanism of elevating the sentiments of the underprivileged (compliance, modesty, forgiveness) over more honest human motivations. (He would most likely be just as disgusted by modern post-Christian social justice warriors who double down on those sentiments and consider Christianity among the oppressive forces that empower some people over others.)
It was the feeling that attempts at social control are never entirely honest. As PeDeLeao points out above, but takes out of context, his aversion is part of an instinct toward authenticity, which does not allow for chosen positions that conform the individual to society as praiseworthy in themselves. (He complains often elsewhere about this instinct as a disease.)
As he lays out in the Genealogy of Morals, Nietzsche sees Christianity as an ingenious attempt to control natural humanity. He later enumerates Jesus as one of the great Creators. He points out that Christianity is a designed psychology that works by inverting one of naive human emotionality's main driving forces -- the feeling we have that links effectiveness and freedom to deserving. It instead links effective control of other people (the rich man) and pursuing one's own ends (unencumbered by God's plan) with selfishness, pettiness and arrogance (through the Lucifer mythology), and ultimately with evil.
As such a carefully tuned manipulation, it could not really be honest at its root, it must actually be based in the desire for order, and the advancement of the species that constructed it.