Not if there actually is a creator. Your premise assumes a priori that there is no God and then tries to speciously inject this assumption into the internal logic of God-based belief systems. If there is a God who gave us these rights, then by definition our rights are not weakened by acknowledging what is then a fact; they are instead infinitely strengthened. On the other hand, if there is no God, then no-one has any rights outside his own ever-fickle mind or that of others because we are then nothing more than atoms and dust just doing what atoms and dust do.
...as not everyone will agree on the nature and preferences of the creator.
The proverbial five blind men feeling different parts of the elephant may have disagreed on the nature of the elephant, but it does not follow from their disagreement that there was no elephant.
Moreover, our courtrooms count as evidence that not everyone will agree on the nature and preferences of any contract. By your logic, we should no longer believe in the existence of contracts or agreements of any kind.
Moreover, people are more fickle with their beliefs than with their contracts.
Not in situations where their beliefs are enforced with the same degree of authority as their contracts -- which was the case for the vast majority of human history.
But rights are not a matter of contract. They follow ineluctably from taking human life as the standard of value.
True in the sense that sunny days follow ineluctably from the weather deciding not to rain that day.
That's all there is to it! Now, you may say that not everyone has to take human life as their standard of value, and you'd be right.
In terms of world history, you are living in the highly aberrational dream world of Western Civilization. 99.999999% of all humans who ever lived did not live in societies where human life was the standard of value.
But the overwhelming majority of people take human life as their standard of value, whether they acknowledge it or not, so this is a much firmer foudation for the Rights of Man than any book or sect.
False: See above -- the overwhelming majority of people do no such thing (SEE: WORLD HISTORY). They take their OWN human life as their standard of value but they have to be taught to apply this to others. The moment it appears advantageous to stop believing all human life is sacred and just do whatever you want, then this "firmer foundation" slips out from under us faster than a Malibu hillside in a downpour. As Doestoevsky said, "If there is no God, then everything is permissible."
An eternal omnipotent God who metes out eternal punishment in the next life creates a major and permanent disadvantage to abandoning the belief that all human life is sacred. There is no alternative to the Judeo-Christian value system that can be anywhere near as effective in this regard.
No God means there is no such thing as "rights"; there is only rearranging deck chairs on the Titanic.
Again, this is an assertion. The concept of "rights" can be accepted without recourse to the Almighty. Indeed, Scripture doesn't address rights as we see them. There was no right to life in the Bible -- God had the Israelites wipe out whole towns. There was no right to liberty in Scripture -- slavery was readily accepted, and anyone not toeing the party line could be, and often were, killed. There was no right to property in the Bible -- the Israelites stole an entire region from the folks living there purely on God's command.
"Rights" are a human concept derived from enlightened self interest.