General revelation is what can be known about God and the moral law by all persons at all times. It is what is available to humans through creation and history. The light of nature is used to understand both general revelation and special revelation (scripture). Reason is called the light of nature because it is that by which we understand. Just as physical light allows our eyes to make distinctions of what is around us, so too reason illuminates our minds and allows us to make distinctions including between God and creation, and between good and evil.
In thinking about God we are distinguishing between God and not God. What immediately stands out in this distinction is the eternal power of God. God alone has existed from eternity. From general revelation we can know that only God is eternal. To identify some aspect of the creation with the eternal power of God is idolatry.
From general revelation we can know that the good for a thing is based on the nature of that thing. So the good for a human is based on human nature. The distinguishing feature of human nature is the attempt to understand, to find meaning in the world. To understand the meaning of the creation reveals the nature of God. And so the good for a human is to know God in all that by which He has made himself known. The moral law knowable from general revelation (and summarized in the 10 commandments) is a description of how to achieve the highest good.
General revelation has been criticized as unknowable or unclear. This criticism says that humans have always had general revelation and if it led to knowledge then humans would now know. The criticism sometimes adds that special revelation is needed because it is a more clear source of knowledge. However, this assumes that humans are seeking. If instead humans are not seeking then their failure to know what is clear from general revelation is an indictment on them and not on what is clearly revealed in general revelation. Scripture has also been a source of great disunity, and so the claim that it is needed because humans have not agreed on general revelation would beg the question. Instead, the reality that humans have failed to see what is clear from general revelation is a witness to the reality that humans are not seeking and are in need of redemptive revelation.
General revelation has been criticized as unnecessary. This criticism says that since scripture gives the only explanation of how redemption is possible general revelation cannot add to this and so is not needed. However, this is a category mistake. It is because humans have failed to know what is clear about God from general revelation that they are in need of redemption. This redemption is revealed in scripture and restores humans to their ability to know God in all that by which he makes himself known.
General revelation has been criticized as allowing in too much. The study of the book of nature can be said to have opened the door to naturalism which is at odds with what special revelation says about origins. Two important examples from modernity are Darwin and Marx. Darwin argued that all speciation can be explained through suffering and great lengths of time. However, this is not only in contrast with the teachings of special revelation, but also what can be known from general revelation about how God made the world very good (without suffering). Marx argued that the origin and source of change in human society is reducible to the distribution of capital. His material dialectic is in contrast not only with scripture but with the truths of general revelation that God is real, God is a spirit (not all is matter), and humans are not reducible to their bodies or material wants. General revelation does not open the door to naturalism but instead sets the context for, and is the source of, the knowledge of God.
General revelation is full and clear. This is in contrast to saying that general revelation is bare and minimal. One example of the latter is that general revelation is only needed to hold persons accountable. Another is that the highest good will be achieved in heaven apart from the works of God. In contrast to these, the works of God in both creation and providence are the source of the knowledge of God. This knowledge of God is inexhaustible. The goal is for the knowledge of God to fill the earth as the waters cover the sea.
The clarity of general revelation sets the context for sin. Sin as the failure to seek, failure to understand, and failure to do what is right, shapes all of the human person and life. The need for redemption presupposes the reality of this sin. Redemption restores us to the knowledge of God as eternal life.