ealgeone:
The Pope was only following the true OT canon of the early undivided Catholic Church, 2 Maccabees, which is in both the Catholic and Eastern Orthodox canon. It is not in the Protestant canon, but that is the minority canon.
People commit sins after baptism and confirmation (Catholic doctrine) or after making the call of the altar, or however you all call it, in protestant doctrine. So the statement you quote should be understood in the Catholic context of what it actually means. These were men, who were of the faith, who had strayed in their adult lives, so fighting a just war to defend Christendom was seen as a sign of repentance.
You’re saying each and every Crusader was a believer? Every one?
Where in the OT does it promise the forgiveness of sins for fighting and dying in battle?