The Connected Planes

Once, Earth had no magic. Humans used technology to overcome their limitations and to combat the natural forces of the world.   And then, one day, a portal opened, and something fundamental changed. Earth was now connected to the magical forces which brought the other worlds together. Humans began manifesting magical abilities. Those who didn't could learn to cast spells. The reach of other planes' deities could now extend to Earth, and paladins and clerics, wielding divine powers, became part of Earth's culture.