The original quote as written by Plath appeared in her review of C.A. Trypanis' volume of poetry Stones of Troy which was published in the Summer 1957 issue of Gemini, pages 98-103. The quote appears on page 102:
A similar sense of fresh, first-hand observation and reaction is revealed in Chartres. Again, we are contemplating a work of art: this time, the architecture and stained-glass windows of a French cathedral. The metaphor-moral is intrinsic to the poem, working back and forth on itself, not expressed prosaically at the close, like the moral of a fable. And in spite of questionable simile 'as free of flesh as sin' (isn't sin, ipso facto, inherent in the flesh?), the words and rhythms of the poem make the church leap alive in colour and light...So, quite possible the quote is taken out of context considering it is a critique of a Trypanis poem. But at least we have the source. This quote was reprinted in Linda Wagner[-Martin]'s Critical Essays on Sylvia Plath (G.K. Hall, 1984) on page 18.
The review -- like all of Plath's book reviews -- remains uncollected.