Probably The Omega Man. Sci-Fi/horror film from 1971 with Charlton Heston in the lead. Most of mankind is wiped out by bio-warfare. Only a handful survive the virus. Most survivors are changed – psychotic and sensitive to light (their eyes are white). The infected gravitate to a charismatic leader who is hell-bent on wiping out the few humans who survived unchanged. The cult members dress in cloaks.
Heston is a scientist who is immune to the virus. He lives sealed off in a building with a lab where he toils to discover a cure for those infected with the virus. But the cult leader doesn’t want to be cured. Each night is a stand-off with Heston fending off attacks by hundreds of the infected.
I wasn’t able to post the image directly, so here is a link to an image at imdb showing some cult members with their white eyes and cloaks.