In the winter of 5733, an unstable man who wanted to convert to Christianity barged into the Rebbe’s room. The Rebbe took the time to talk to him, and even begged him to stay for the Rebbe’s birthday.
Reb Shlomo Zarchi relates:
During the winter of 5733 (1973), an erratic individual found his way to 770. This person, who was obviously suffering mentally, would spend many hours in the zal of the yeshiva bochurim.
Once, during a yechidus night, this person barged into the Rebbe’s room uninvited. The mazkir immediately tried removing him, but the Rebbe stopped the mazkir, and spent some time with him.
At some point this fellow decided to convert to Christianity r”l. The Rebbe sat with him for a while and invested energy to convince him not to do it. At the Parshas Para farbrengen, the Rebbe related the story of the Mezritcher Maggid who stopped a disciple from converting by saying a Torah on the posuk “Nefesh ki sechta.” As the Rebbe said the story, the Rebbe began weeping.
A while later, he informed the Rebbe that he would be leaving to accomplish his plans r”l, and thanked the Rebbe for all his help…
But the Rebbe didn’t give up. In a handwritten note the Rebbe beseeched him to stay in 770 and spent the yomim tovim with him as well as his own birthday.
Eventually he indeed dropped his plans.
(As heard from Reb Shlomo Zarchi; Igros Kodesh, Vol. 28, p. 18)