atheist argument against the widespread witness of angels

[samiam2013]

I have always doubted the existence of a god or gods, but recently have seen more than before than it makes sense that there is no god. I do however, have problems with one thing: the acknowledgement of my grandfather to having seen angels just before both my mother and his sister dying. Is there a solid argument for why seeing angels is a delusion? (he was illiterate, believed in god, and went to church while living a simple life as a carpenter)