First you need to see The Robe, which was one of the first big CinemaScope productions, since this movie is the sequel. The Robe was based on the novel by Lloyd C. Douglas, who decided to give a fictional answer to the question of what happened to the soldier who won Christ's robe in a dice game at the foot of the cross.
In the movie version of the book that soldier is played by Richard Burton. The movie must have been a big hit, since a sequel came out not long afterward. It's not based on a novel, but some of the characters from The Robe return. Not Burton, as you'll see in the opening of Demetrius and the Gladiators, which begins with the last couple of minutes of The Robe, as Burton and Jean Peters are marched off to die. Peters hands the robe to a man in the crowd and says, "For the big fisherman," Peter, of course, played in both films by Michael Rennie.
The mad (boy, is he!) emperor Caligula (Jay Robinson) gets the idea that the robe is magical and tries to force Demetrius (Victor Mature) to tell him its location. Demetrius refuses and is sent to fight in the arena, where he fights but won't kill. He's visited by Lucia (Debra Paget), who's assaulted by the other gladiators. Demetrius prays to God to save her, but she falls (apparently) dead. Demetrius renounces God and Christ and goes out and slaughters the gladiators he blames for Lucia's (apparent) death. Caligula appoints him to the Praetorian Guard. Further complications ensue, some of them involving Susan Hayward, back from The Robe and totally glamorous. With her and Debra Paget, you just can't go wrong. And besides those two and the others I've mentioned there's a wonderful supporting cast: Richard Egan, Ernest Borgnine, Anne Bancroft, William Marshall, and even Julie Newmar in a small role.
The '50s were a great time for big-budget biblical spectacles, and this is one of the better ones, with lots of action, a good story, and that dandy cast, not to mention CinemaScope and Technicolor. [SPOILER ALERT] As you'd expect, Demetrius is reconverted to Christianity by the end, and lots of people even live happily ever after. Not Caligula, though.
The entire movie's available on YouTube, but this is one better seen on the big screen. The bigger, the better, not that there's any chance of that.