“However, Coco noted that Koenig also urged the Holy See to not make public what he was revealing because he feared for his own life and the lives of the resistance sources who had provided the intelligence.”
Koenig, for the 99% of people who don’t read past the headline, is the author of the letter in question.
You can take action without revealing your sources. Journalists and police officers do it every day. Politicians and businesses do it all the time. Yet the all powerful Catholic Church did nothing.