We can rationalize what's in this world. The world exists, therefore its creator exists. By rationalizing his creation we can rationalize the creator. Therefore God can be understood by rationality.
This argument hinges on the assumption that the world was created which is not a proven claim.
You can not necessarily come to conclusions about a maker by looking at the product. Looking at Picasso's paintings will not tell you what kind of person he was. An alien wouldn't even see that Picasso was a human.