I'm puzzled by your statement here. From what I've read natural law theorists believe that in every person there exist some sort of God instilled moral law. What you're saying, if I'm interpreting your above statement correctly, that although man is fallen God instills natural law in man through the church. Is this correct? These seem to be different definitions in how natural law exist.
Let me clarify even further: I agree that there is a moral law, or at least the first principles of moral law which are accessible to every person who uses reason rightly. I do not think that this means every person will reason rightly.
I do not take the fallen nature of mankind to be such that it precludes individual innate knowledge of some first moral principles. In my understanding, The Fall precludes mankind's access to salvific grace in such a way that only God can restore that access.
I don't see how the Fall precludes man's access to all moral knowledge--though I will grant that the Fall significantly clouds his reasoning. Because of his clouded understanding, each man needs to check his reasoning against the reasoning of those wiser than himself and against the wisdom of the church.
Man using his God-given intellect can grasp the first principles of logic and of math. Why then not the first principles of morals?