Full Idea
Schopenhauer's interpretation of the in-itself as will was an essential step: but he didn't know how to deify the will, and remained caught in the moral, Christian ideal
Gist of Idea
Schopenhauer was caught in Christian ideals, because he didn't deify his 'will'
Source
comment on Arthur Schopenhauer (The World as Will and Idea [1819]) by Friedrich Nietzsche - Writings from Late Notebooks 9[42]
Book Reference
Nietzsche,Friedrich: 'Writings from the Late Notebooks', ed/tr. Bittner,Rüdiger [CUP 2003], p.149
A Reaction
Intriguingly, this seems to suggest that Nietzsche consciously sought to replace the absence of God with the human will, which strikes me as an odd, and very nineteenth century, idea. Loss of religion bothered them a lot.