Natural Law and the Ignorance Defence

We say that ignorance of the law is no defense. For example, if a man is charged with murder he can't claim that he didn't know it was wrong. This is based on the idea of Natural Law, that the dictates of God are written on the conscience. But our society is moving away from this, towards arbitrary law based on a Constitution. So... if we no longer believe the requirements of justice are written on our hearts, does that mean that ignorance is now a defense?