This, too, we reconcile by John M. Ford. I read it in Analog May 1976, which appears to be the only place it has been published.
The protangonist, Easton, is a Remember and as you say he is asked to investigate the monk Brother Peter who was tortured to death to find out if he ascended to heaven. As you say, he finds that Brother Peter was damned by a single sin.
It turns out that Easton wasn't the first Remember asked to investigate this. He was preceded by Falk who died during the experience. Easton doesn't die but is traumatised by the experience. He lies about Peter's ascension:
The Abbot took me into his office, alone, and asked if Brother Peter had been received unto God. And I said yes, which was the truth. Then he asked for details, and I began to lie.
I described a magnificent holo show production, starring the arch angel Michael, with a heavenly host of thousands and a guest appearance by the Christus himself (whose mighty visage was suffused with pure light and therefore invisible). It was the most outrageous fabrication of my life, and it is going to make Brother Peter the first off-Earth saint.
The story ends:
I called Teller and told him to get me out of there. He got. I'm on Alte schloss now, an Agency world where I can have a wipe done without any questions. This one is going to be total - no souvenirs of Brother Peter. There are too many people who know memory is holographic, too many who'd like to hear about the ascension of St. Peter the Vialandran.
What do I say? Peter/I could have been hallucinating, under the weight of pain and poison... There might have been nothing for us to see except black infinity. Or there might have been an equally black truth.
How do you explain eternal punishment for a single sin? How do you, for that matter, account for the existence of the serpent in Eden? Or the great deliverance from Egypt - to seven Israb wars? How do you reconcile St. Sebastian?
As I say, it might have all been a nightmare. But there is Falk ... he was a Class One Rememberer, by definition not a coward. But so am I, and I do not have the courage to offer the universe a choice between the empty finality of no God - or a Supreme Torturer who lives on the screams of souls and whose true prophets are being exterminated.
Hurry up with the wipe, before I jump too.