Nehtdx

I read this long story (novella?) a couple of decades ago, I believe, and probably in one of the SF magazines. There's a good chance this was in F&SF of the time.

It involves a priest who was an ex-soldier of WWI. He resides in a abbey that conceals a room that gives accurate visions of the future to one person in a generation who sits in it and dreams. He was that person for his time.

The next person in the line of oracles, so to speak, is a young woman who is drawn to that abbey. Stuff happens, the priest resists telling her about the room, but in the end she finds out -- by going into the room and seeing the world ending in nuclear war.

She accosts the priest and asks him why he didn't do anything, why didn't he use the room to guide humanity away from that terrible ending he also knew about.

His response was, "I lost my faith in humanity in the trenches of Verdun." (Obviously I am misremembering the exact quote as googlemind knows nothing about that.)

Does that ring a bell with anyone?

44 years, not a couple of decades. A powerful story, for me to remember it after reading it just once.

This is "The Custodians" (1975) by Richard Cowper.

You've remembered the gist of it quite well - the abbey, the prophetic room, the young woman, the fact that it appeared in F&SF.

The quote isn't quite as you recall, nor is the last line, though it's near the end. It's not a spoken line but ruminations in the mind of the priest, Spindrift. It actually reads

...Spindrift's God had died in the mud of Ypres.

or maybe it's the line

...some vital spark of humanity had been extinguished far back in the
blood-stained ruins of 1917.

But the point is as you remembered, this prevents him from taking any action to ward off Armageddon.