He had a strong voice, and a muscular build, which made him popular for the type of roles he played. His voice made him popular as a narrator as well.
I don't recall his days as a civil-rights activist. His political activities came to my attention in the '80s, when he was a vocal supporter of Ronaldus Maximus (whom he knew from his film days) and later as the National Rifle Association spokesman.
Many of his films are classics, and he set the standard by which some characters are measured: Moses, Ben Hur, etc.,. In those films, however, the acting was secondary to the spectacle. He did perform in Shakespeare's Hamlet, but I don't know how well he did since I didn't see it. (Isn't Shakespeare the barometer used to guage one's acting ability?)
Few performers can match the roles he's played, from God to Moses, Michaelangelo to Hamlet, the last man on Earth to a skin-clad astronaut wrestling with monkeys. Quite an achievement, IMHO.
Quod scripsi, scripsi.