SCRIPTURE: Rejection of Natural Law Leads to Depravity

It is Catholic teaching that God is able to be known through reason alone, without the assistance of Divine Revelation. The doctrine, officially promulgated as dogma by the First Vatican Council, is sometimes misunderstood, but what it means is that man has been given sufficient intellect to perceive through the created world that there is indeed a Creator, without need of recourse to some divine oracle, such as that of Mt. Sinai (not that intellect alone, without grace, can put someone into a relationship with God.)

St. Paul the Apostle expounds on Natural Law in
his epistle to the Romans

Man can know by his own faculties (principled upon that he is made in the Image of God, and that God's very Logos is "the light which enlightens every man" as St. John puts it) that there is indeed a God and that there a true moral standard imposed on mankind, to which mankind must submit. This, in a word, (the word of the Catholic tradition to be precise,) is Natural Law. It is the apparent principle within creation which makes man able of perceiving philosophical and moral truths concerning God and the expectations He has enjoined upon his creatures.

Were there no Natural Law, no law of God which shows "that what the [Mosaic] law requires is written on their hearts" (Romans 2:15), what basis could God have for judging Christian and non Christian alike by the same standards of justice? Were they not manifest in some natural way to the natural man, who lacks the supernatural initiation into God's covenant. Thus, the Natural Law is binding upon all mankind, Jew and non-Jew, Christian and non-Christian.

The result of rejecting the Natural Law, however, is a plunge into intellectual, emotional, and spiritual darkness. Intellectual darkness, in that their sin eclipses their reason and they can no longer perceive these truths knowable to them by the principle of the imago Dei. Emotional darkness, that they become automated by their passions (anger, lust, etc) and are plagued by an assault from them. Spiritual darkness, that they become in bondage to and subjugated to dark spiritual forces of hell, giving themselves over to Satan.

And this is exactly what has happened throughout human history. Through the inheritance of original sin, mankind has regularly rejected the natural law and given himself over to evil. Two passages in particular will suffice to demonstrate this, though others could be cited. One is from the Old Testament, and the other from the New.

Wisdom 13:1-9

For all people who were ignorant of God were foolish by nature;and they were unable from the good things that are seen to know the one who exists,nor did they recognize the artisan while paying heed to his works;but they supposed that either fire or wind or swift air,or the circle of the stars, or turbulent water,or the luminaries of heaven were the gods that rule the world.If through delight in the beauty of these things people assumed them to be gods,let them know how much better than these is their Lord,for the author of beauty created them.And if people[a] were amazed at their power and working,let them perceive from themhow much more powerful is the one who formed them.
For from the greatness and beauty of created things
comes a corresponding perception of their Creator.
Yet these people are little to be blamed,for perhaps they go astraywhile seeking God and desiring to find him.For while they live among his works, they keep searching,and they trust in what they see, because the things that are seen are beautiful.Yet again, not even they are to be excused;for if they had the power to know so muchthat they could investigate the world,how did they fail to find sooner the Lord of these things?

Romans 1:18-25

For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse; for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man or birds or animals or reptiles.
Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed for ever! Amen.

As can be extracted from these Texts, mankind has been given through creation a window into which they can see some general truths of the Creator. The sin of idolatry arose when mankind forfeited those truths for truths of his own making, which are not truths at all. Man "exchanged the truth about God for a lie."

This darkness obscures man's ability to perceive this basic truths of God, and in this darkness, he becomes a slave to lies, to falsehood, and to evil. This is one of the factors that the Blessed God of Heaven sent Son into the world, to be a "light of the world." The very Logos of God became Incarnate in the Person of Jesus Christ, and He came as "the Way, the Truth, and the Life." (Jn. 14:1) This truth, the truth of God, is the truth that sets us free.

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(Side tangent: this biblical principle of Natural Law is actually why the Church has looked favorably upon the works of pagan philosophers. It is not that Greek philosophy is on the same moral and spiritual authority as Sacred Scripture or Divine Revelation, but that it vouches for the Biblical assertion that man can know certain truths pertaining to God without direct Revelation. Men such as Socrates and Aristotle rightly rejected the local Athenian cult and its polytheism, perceiving it as intellectually crude and insufficient to explain the origins of the universe. Through their studies, discipline and contemplation, they recognized the need for but One universal principle: an unmoved Mover, a first cause, etc.)