This person has basically dodged the question on the concept of "God's law" by making generalised comments about Greek and Roman society. Ironically, Christian Philosophical thought actually owes a lot to the Greeks Plato and Aristotle, with Augustine and Aquinas drawing heavily upon both of these in an attempt to make the logical thinking of the ancients compatible with Biblical teachings. It is from Aquinas that the concept of natural law is derived (or "God's law" as it is called here) in the first place. I.e this person has attacked the very societies from which his argument was derived (with the Romans translating texts such as the Republic into Latin).
Plus, there are problems with this argument:
If Natural law is unchanging, whilst human law is constantly changing it raises the question:
Is natural law right because God says it is right? or is Natural law higher than God, and he is therefore bound by something beyond himself? If it's the latter then the Western concept of God falls apart because it means God cannot be a transcendent entity.
If it's the first then how is it different from human law? Natural law becomes something which is no longer fixed, but chosen, thus leaving them open to change, which defeats the very point of having unchanging laws. Therefore, the concept of a fixed morality actually doesn't fit very neatly with the Western notion of God (Judaism, Christanity, Islam- they all share the same routes)
Basically, the concept of natural law does not require there to be a God, in fact, it is a far more coherent concept when secularised, which is exactly what has happened, has anyone ever heard of ideas such as universal human rights? In short, using the "God's law" argument is all very well, so long as you can back it up; which is exactly what has not been done here.
EDIT: we need more Augustinian Christians out there, they'd stop meddling with legislation so much.
