The idea could be that enemy morale is hurt if they think they can’t get into heaven
You know how Catholic used to believe that unbaptized babies go to purgatory until recently? Seems like the same kind of thinking.
pedantic ex-Catholic voice Actually, the unbaptized babies were supposed to go to limbo, not purgatory, which is worse. In purgatory you can work off your sins and eventually get into heaven; in limbo you are just stuck for all eternity doing nothing. That’s the horrifying bit that the Church no longer officially endorses
Of course they don’t want to get shot and killed, but making them think they’ll be damned and killed might make them more reluctant to fight . That’s what I think the logic was