Reformation-era debates regarding divine revelation focused on the authority of Scripture and its relation to the authority of the church and the authority of church tradition. Though the Reformers and the Roman Catholic Church would finally disagree with respect to the final authority and sufficiency of Scripture, there is one aspect of divine revelation on which they did find a large measure of agreement. Both Roman Catholic theology and Reformation theology confess that God reveals Himself in His creation.
In theological categories, we speak of God's revelation of Himself in the created order as natural revelation. This is in contrast with special revelation, wherein God speaks directly to His people via a prophet or an Apostle. God's revelation of Himself in natural revelation is less direct and is addressed not to a specific person or community but rather to humanity in general. For that reason, natural revelation is also known as general revelation.
We say that natural revelation is less direct than special revelation because in natural revelation, the Lord does not disclose specific truths about salvation, specific plans for individuals, or anything of that nature. Instead, He reveals Himself and His attributes in a general way. Essentially, God reveals Himself through natural revelation as the Creator of all things. As we see in today's passage, the heavens themselves proclaim that they are the handiwork of a personal Creator (v. 1).
Psalm 19 emphasizes the universality of natural revelation. There is no place on the planet where God's natural revelation does not proclaim that He is and that He has made all things. Romans 1:20–21 fleshes out this revelation a bit more, explaining that natural revelation proclaims God's power and that we owe Him honor, thanks, and worship. It does not tell us everything there is to know about the Lord, and we can hardly build an extensive theology based on natural revelation. But it does tell us enough—that we are creatures and that there is a Creator to whom our worship is owed.
At the end of the day, no one can be a true atheist because natural revelation is so clear. John Calvin writes, "[God's] essence, indeed, is incomprehensible, utterly transcending all human thought; but on each of his works his glory is engraven in characters so bright, so distinct, and so illustrious, that none, however dull and illiterate, can plead ignorance as their excuse" (Institutes 1.5.1).
Natural revelation is limited in its scope, but that does not mean it fails to achieve its purposes. God reveals Himself in nature so that no one will be able to plead ignorance of His existence on the last day. His message gets through, and we can appeal to creation as proof of His existence when we are talking with unbelievers. Let us not be afraid to use God's natural revelation to point others to Him.