Yet the postmark read today; the letter had been mailed, in fact, only about six hours ago, just in time to catch the dawn missile to Naples. Michelis had been sitting on it—or perhaps adding to it, but the facsimile process and the ensmallment, together with Ruiz's gathering eyestrain, all conspired to make it impossible to detect differences in the handwriting or the ink.
A Jesuit exobiologist on a curiously peaceful and civilised planet begins to worry about the theological implications of his discoveries — and his suspicions are confirmed when the locals send an ambassador to Earth. I thought the alien civilisation was remarkably interesting, although for the purposes of this story it's really just background; wish there was more written about it.
While the writing hasn't dated badly at all (I would have assumed it was mid-70s, rather than 1958), the odd plot detail trips it up: the idea that ontogeny recapitulates phylogeny is key to the theological crisis the book describes.
I read this in the "After Such Knowledge" paperback compilation, which I wouldn't recommend buying because it has page sequences randomly garbled at various points throughout the book.