And never the twain need meet. I can be the perfect gentlemen and citizen, and yet deny the existence of HaShem. I can be the most ethical and upright of individuals without a G-d. The Torah is telling us that this is not true. If the whole system of laws is legislated by man, then just as man can create laws, man can change laws.
Man can legislate one thing today and can legislate the complete opposite tomorrow.
Without a higher authority, there are no laws that can not be changed. If man is the creator of the system of laws, then there really are not any laws between man and man either. Therefore, after the Torah tells us the laws of the Blasphemer, the Torah begins to establish laws relating to man's relationship with his fellow man. That is the only way that the system can work.