We can do this without blasphemy because it is our own nature, not God, that is the source of our morality.
And the source of my nature and ability to reason is...???
I'm sure these fellows would tell us the source is a random interaction of the elements of physical reality.
You know what's really scary about these whackos? They may insist on a world that began with absolutely no meaning of any kind, but they seem to believe that the world is actually gaining meaning as it evolves, and they believe that they are the ultimate authorities on the authentic evolving morality. It's almost the old Hegelian position of the world creating G-d (chas vechalilah!) rather than vice versa. They detest and ridicule "intelligent design," yet they use the language of design continually, almost delighting in flaunting it. The more they insist on the meaninglessness of the beginning of the creation process the more they use the teleological language of "progressive" vs. "reactionary" in describing what the meaningless world is becoming. And it seems to be becoming a totalitarian planetary society that will brook no dissent, all the while attacking "religious fundamentalists" for brooking no dissent! Apparently the absence of meaning at the beginning satisfies these people that no totalitarian "omega point" can be unjust or oppressive.
It's this precise issue of "meaninglessness acquiring ultimate meaning" that exposes atheist humanists as bizarre new age mystics parading as scientific materialists.
"and they believe that they are the ultimate authorities on the authentic evolving morality"
Genesis 3
3:1. Now the serpent was more subtle than any of the beasts of the earth which the Lord God had made. And he said to the woman: Why hath God commanded you, that you should not eat of every tree of paradise?
3:2. And the woman answered him, saying: Of the fruit of the trees that are in paradise we do eat:
3:3. But of the fruit of the tree which is in the midst of paradise, God hath commanded us that we should not eat; and that we should not touch it, lest perhaps we die.
3:4. And the serpent said to the woman: No, you shall not die the death.
3:5. For God doth know that in what day soever you shall eat thereof, your eyes shall be opened: and you shall be as Gods, knowing good and evil.
3:6. And the woman saw that the tree was good to eat, and fair to the eyes, and delightful to behold: and she took of the fruit thereof, and did eat, and gave to her husband, who did eat.
We Catholics call it Original Sin, and it is with us to this day.