Pope Francis tweeted on September 21st,
Sermonem confero cum aliquo sincerum tunc tantum agnosco illum esse donum Dei mihique aliquid pretiosum dicturum.
Here is my translation. (Credits to Keith Massey for his help.)
I engage in a sincere dialogue with someone only after I recognize that person is a gift of God and about to say something precious to me.
My confusion is with the phrase tunc tantum. I recognize tantum as an adverb meaning "only", but I'm unsure of how to translate tunc alongside of it. I am accustomed to tunc meaning "then", but in this case could it possibly mean "when"?
I checked the Lewis and Short dictionary, and couldn't find this sense of tunc. But it seems to be the most appropriate translation. Is going from "then" to "when" not such a big stretch after all — am I merely nitpicking? Or are there dictionaries that document this sense of tunc after all?