Can we define God? I supose you will admit that it is equivalent to say Can we know God? If we accept that God exists, we sure will admit that the universe is His creation. Therefore, the progress in the knowledge of the creation, should enhance the progress in the knowledge of the Creator. So, in a way our knowledge of God should also evolve, and in fact I think it evolves, (for those that accept its existence, of course). Nevertheless, I think that we should admit that our reason may be not "equipped” to fully understand God.