I think the letter was written by Koob to prove he wasn't a suspect.

Father Koob would say he handed it over willingly. The Keepers is a documentary that explores the circumstances surrounding the unsolved murder of Sister Cathy Cesnik, a well-loved nun who taught at Baltimore's Archbishop Keough High School in the 1960s.In their quest for answers, the series also blows the doors off of the rampant sexual abuse by Father A. Joseph Maskell and the potential cover-up by the church and law enforcement. The Letter Dated November 3, 1969 “My very dearest Gerry, ‘If Ever I Should Leave You’ is playing on the radio. May 31, 2017 Can someone shed some light on the "letter" that Sister Cathy supposedly wrote to Gerry Koob on November 3rd (a few days prior to her disappearance) saying to the effect that she has made her decision to go on with her nun life and that she's ok loving him "within herself."

I’m all curled up in bed. Father Koob insisted that he and Sister Cathy were just good friends, strictly platonic but then Detective Captain Louis “Bud” Roemer visited Father Koob at the Priest’s quarters in Manresa and discovered the letter. I'm sorry if I denied you and chose my faith I wouldn't be talking about wanting you in me. What poet would write about their period and tell the person that they didn't want to marry "i love you".