I grew up in a Catholic household in a predominantly Christian part of the rural Midwest. It seems like there's a church on every street corner of my hometown. The Christian world view was part of our culture and our identity. As a young boy interested in science, it's therefore no surprise that one of the sources of information that I was given was a cassette tape that talked about all the evidence of God's work in nature. I listened to this cassette repeatedly.
It's been many years, so I only really remember one particular part. The preacher (I assume he was a Christian preacher or minister that made the tape) was talking about the fact that when you look at the branch of a tree, you'll notice that the side branches do not grow out of that branch in a random pattern. In fact, they follow a repeating pattern where, for example, every fifth branch is pointing in the same direction. This order was evidence of God's hand in the creation of a tree. God made the tree with a certain logic, and this was evidence of the creator's work.
For a Christian, or any believer, the existence of a god is assumed. It's part of the fabric of the Universe. As such, any observation of order in the Universe is considered evidence of that god's existence. The problem with this argument is that it is circular. There is an assumption that a god exists with certain properties and any observations that are made are taken as evidence that the god exists. The argument is "God exists. God creates order in the world. I see order in the world. Therefore, God exists." The argument implies that the only way there can be order in the world is because God causes it. Thus, any order is evidence of God. It's not a valid argument to assume something is true and then use that assumption to argue that it is true.
Another problem with this is that the selection of evidence is arbitrary. Observations that demonstrate their claim is taken as evidence; any observation that contradicts it is ignored. If something is true, then there shouldn't be contradictory evidence. If there is, then the thing can't be true. In reality, if there is contradictory evidence, perhaps you don't have to throw out the idea altogether, but you need to revise it to account for the contradictions.
Let me give an example of something that is also a part of the fabric of the Universe - gravity. Gravity is a basic property of matter. Anything with mass will attract anything else with mass. On the earth, we see this in the fact that things fall to the ground. The gravity of the earth is so large that we don't normally notice that all the smaller things are attracting one-another, but this fact has been verified in laboratories and in outer space. There are other known forces that cause objects to be attracted to one-another, including the strong nuclear force, the weak nuclear force, and the electromagnetic force. When we remove the effects of those other forces, we still find that gravitational force is there. Similarly, if we see two objects being attracted to one-another, we don't assume it is because of gravity. We look at all the possible explanations and then decide if it is because of gravity.
We didn't start out with some theory of gravity from the start and afterwards looked to find evidence of it. On the contrary, we observed that things fell to earth, and we wondered why. As we studied this more and more, scientists like Sir Isaac Newton and others did experiments to measure the effects of gravity and to come up with a description of how it works. Over the centuries, our understanding of gravity has grown, and it helps us to understand how the Universe works. Everywhere we look, we find that the effects of gravity are there. There are no contradictory observations. The interesting thing is that we still don't know exactly what causes gravity. I think scientists are still working on trying to explain how gravity is related to Relativity, as Einstein described it, or with quantum mechanics.
The argument about gravity is like this: "We observed that objects attract one another. The force that we call 'gravity' explains that attraction. Therefore, gravity exists." This is not a circular argument. We don't assume that gravity exists at the start. The existence of gravity is the conclusion.
If we wanted to follow the same process for proving the existence of a god, we would start with our observations. What are these observations? We see order in nature. Is the existence of a god the best explanation for this order? Scientists have observed true randomness on the quantum scale. How does an orderly creation explain this? We see "good" things happen. Is the existence of a god the best explanation of this? We see "bad" things happen. How does the existence of a god explain this apparent contradiction?
The more we know about nature, the more we find that there are better, simpler explanations for the apparent order in the universe than "god did it'. For the example of the regularity of the branches in a tree, we know that natural selection would have favored trees that can position their leaves in locations that capture the most sunlight. A tree that grows branches in regular intervals, thus filling every available gap, would collect more sunlight than one that grew branches at random. Random branches would likely leave gaps (missing the opportunity to capture sunlight), and there would be branches that block other branches (wasting energy spent on growing the branches that are blocked). Over the millions of years that trees have been evolving, the individual trees of a given species that did the best at collecting sunlight would be more successful. They would have more energy from the sun that they could use to produce more seeds. Over the generations, their offspring would be more successful than the offspring of trees with worse branch placement. These "orderly branch" genes would come to be dominant in the population of trees. Eventually, all the trees in the population would have these orderly branches. Thus, it's no surprise that all trees of a species show the same order to their branches. They are all descendents of trees that developed a particular genetic mutation that allowed them to have more orderly branches than the trees before them.
The existence of orderly branches in a tree is evidence of evolution. Like gravity, evolution is a universal property that we have observe everywhere we look. It is completely consistent with all the observations we make, and it accurately predicts what observations we will make in the future. We continue to learn more and more about evolution as we gather evidence. When we find evidence that appears to contradict evolution, we generally find that the evidence actually clarifies our theory of evolution. If we ever found evidence that truly contradicted evolution, we would be willing to throw away that theory in favor of one that explains all the evidence. This is how we learn about the world. This is how we increase our knowledge of the Universe and better understand our place in it. This is how we use evidence.
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