The Renaissance may have started off the process of science as we know it, but Galileo still had much resistance to his discoveries. We're all familiar with how he was tried before the Inquisition for saying the Earth went around the Sun. What many people are less familiar with are his discoveries using a telescope, which were some of the reasons for his support of the heliocentric model. Contrary to popular belief, Galileo Galilei ("Galileo of the Galileo family") did not invent the telescope. He heard about others inventing it and using it to see distant people and ships, and based upon a description of the telescope he built his own, better, version. He was also the first recorded person to turn his telescope towards the heavens.
Once looking up, Galileo discovered some amazing things that shook his faith in a perfect orderly universe, one in which everything in the heavens was an unblemished sphere orbiting around the Earth.
- Everyone already knew the Moon had phases and varying colors on its surface; Galileo also discovered that it had craters, and thus was a blemished sphere.
- Venus also went through phases like the Moon, in a specific pattern indicating it orbited around the Sun rather than the Earth.
- Jupiter had four moons circling around it, rather than the Earth. These are now called the Galilean moons, and are (from inner out) Io, Europa, Ganymede, and Callisto. We also know today that Io has sulfur volcanos, while Europa's surface is water ice with liquid water underneath.
- Saturn had some weird blobs sticking out from the sides of it. Galileo's telescope wasn't good enough to resolve what they were, so it looked kinda like a teapot with handles, or ears (think Ross Perot), or maybe it was three bodies all orbiting the Sun together. Today we know it's Saturn's rings, and in fact all the gas giants have rings though they're difficult to see from Earth. Further evidence of imperfection.
- Galileo did one of the forbidden things in astronomy: he turned his telescope towards the Sun and looked through it at the telescope (without any filter). This is STUIPD because all the extra light that the telescope gathers is now focused on your eyeball - remember being a kid and frying ants with a magnifying glass? That's what he was doing to his eyeball. I had a prof once who said "you can look at the sun through a telescope unprotected twice - if you're stupid enough to do it again with the remaining eye." That said, Galileo luckily didn't immediately burn out his eyes (though he was blind in later life), and saw the surface of the Sun had sunspots. It was not a perfect unblemished surface.
Another fact actually further belittles Galileo's achievement here: the Chinese discovered sunspots sooner, it's just that Europeans didn't hear about it. And they were smart about it - they watched the Sun's dim reflection in muddy puddles.
Seeing all these amazingly imperfect things through his telescope, Galileo, as a good scientist, shared them with the scientific community. They did not believe him. With the poor grasp of science that people had at the time, they believed that either Galileo built a trick telescope, or that there was some supernatural element to the telescope that was deceiving them all. They were not yet able to distinguish where science left off and other fields began. Ancient philosophers in Greece distinguished even less between fields than Renaissance people - the same individuals were philosophers, mathematicians, astronomers, astrologers, physicists, theologians, politicians, and orators.
Today as our knowledge expands we separate the subjects more and more, up through the generations-long creationism/evolution debate. Scientists try to draw a fine distinction between science (searching for natural explanations) and religion (searching for supernatural reasons). The only way ID could come close to science would be if its supporters said that the designers were some form of alien life, rather than the biblical God. However, once they start saying the universe and all life in it was designed by aliens, it becomes unprovable and untestable with current technology, which plants ID firmly in the field of pseudoscience. ID is either religious creationism, or pseudoscientific hokey.