One must believe in a deity in order to be a Mason, heartwood.
So, strike the “atheist.” Your grandfather was agnostic, believing in “a” god, just not necessarily any preconceived notion of that god.
My grandfather turned down a scholarship to Harvard (for the ministry) because he didn’t believe in God. He raised my mother and her siblings not to believe in God.
He was perfectly capable of prevaricating about his beliefs in order to have the fun of joining a secret society, although to have done so for a scholarship went too far for his occasionally elastic principles.