Well, you can look at it two ways:
1) Either the gods are real beings, powerful and on a higher level than you or I. In this case, you would not be able to create a god.
2) Or, ancient people took forces that they could not understand and broke them down into manageable chunks, gave them a personality, and worshiped them. Over time, with all of the energy the worshipers were putting into the gods, they took on a life of their own. So, under that theory, yes, you would be able to. But it would only be as powerful as what you put into it. When you compare that to, for instance, working with Egyptian deities that have been worshiped for thousands of years, it seems kind of pointless.
As always, just my thoughts.