The messianic aspect of Superman has always been part of the Superman mythos.
Not really, that element only really sprang up after the Donner flicks. Prior, he was the thirties aggressive champion of the poor who fought wife beaters, child abusers and so on. Then the forties and fifties he was the smiling patriot( that's when the famous slogan "Truth, justice and the American way!" popped up) that wasn't afraid to choke fuhrer Hitler to the silver age's overpowered asshole who wasn't afraid to give Lois a spankin.' and could hold multiple planets. He's been through a long metamorphism, and the "extraterrestrial Christ" figure was an interpretation that was seeded and grew from the influence of the Donner movies. The hope and inspiration aspect of Superman has always been a cornerstone of the Superman mythos.