What can sometimes be confusing is that light does change it's speed depending on the medium through which it's travelling. That's why we see diffraction in glass, and why prism make such pretty patterns. The most important notion in Physics, which Eistein realised in the early 1900s, is that the speed of light in a vacuum [which makes up most of space], is constant.
This doesn't just mean it stays the same wherever you measure it in the universe. Oh no.
Imagine someone throwing a cricket ball at you. Say they throw it at 10 metres a second. Say it takes 1 second to reach you. If you wanted to catch it in less than a second, you would run towards it. You running would make the speed of the ball faster RELATIVE to you, and you could catch it, say 0.8 seconds after it was thrown.
Light doesn't do this. It's crazy. Say someone shines a torch at you. Say the light takes a nanosecond to reach you. Now you run towards the light, or run away from it. You can even get in a spaceship and zoom off at millions of km per hour. The light will still take exactly one nanosecond to reach you. The speed of light is constant. Practically all of modern Physics depends on this.
Now for why light bends as it travels through space. This is also down to Einstein. He was after all, a rather bright chap. His 2nd Theory of Relativity, General Relativity, brought gravity into the equation.
Imagine space as a great big rubber sheet. Now chuck a bowling ball into the middle to represent a massive object like a star. The sheet will bend under the ball, creating a well or curved surface. Now take a small ball bearing, to represent light. Ping it across the sheet at a good speed. If the sheet was flat, without the bowling ball, the bearing would move in a perfectly straight line. With the bowling ball, the bearing will move in a curved path, that is it bends as it passes close to the "sun". This is what happens with light. It's been experimentally verified, and is one of the most successful scientific theories ever.
Using this, you can search for matter in the universe, as well as things like black holes. If you see light bending when it shouldn't, there's likely to be a big massive object doing the bending.
As for the notion of God being light, that's an interesting one. But i've got to go and revise. Less science, more religion in tonight's posts!
Cheers
Nick