Respected brother Iftekhar, Salam o Rahmat!
A better term would be NON-MUSLIM. "Kafir" like 'infidel' sounds derogatory and mostly the word is used inaccurately.
A Kafir would be one who conceals the Truth or rejects Islam after it has been decently conveyed to him.
A gentle correction: You wrote, "..... interfaith activities from the last 35 years."
Make it: 'activities for the last .....'