Is there a God? How can we know?
In an attempt to answer this question, I propose looking at relatively simple argument, namely design.
At its core, the design argument is based on 2 propositions. First, we are capable of recognizing design. Second, if something is designed it has a designer.
This first point doesn't seem too complicated. Recognizing design is something we do daily, though it is likely so intuitive that you don't really ever think about it. For example, if you were at the beach and saw a sand castle, would you assume that someone designed and built it or that the natural processes of water and wind erosion created it? Would you really need to stop and think about it? Of course not, you immediately recognize design. In part because natural processes do not result in more organization but less.
For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made.
The second proposition seems even more straight forward. If that sand castle was designed then someone must have designed it. Now, this doesn't assume is that you can identify who the designer was - it simply allows that you can recognize that someone, some intelligent agent, must have done so.
Using this example, you could confidently say that the sand castle was designed and that some intelligent agent designed it with out having any evidence beyond the existence of the sand castle itself.
So how do we apply this to God? Our universe, our world, our bodies are all incredibly complex and highly organized. Even by simply looking at the human mind, we find it is more complex and powerful than any computer man has been able to create (for example, try programming the logic to recognize design!). We are much more complex than any object made by man and therefore, parallelling our sand castle example, our very existence is evidence enough to prove we are designed. And, as our second proposition states, if something is designed it must have a designer. We call this designer God.
As Christians, we would look to the letter Paul wrote to the Romans in 57 AD for further support of this argument. He wrote: "For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse." (Romans 1:19-20 ESV)