Recovering the distinction between sacred and secular will not solve all our problems but, like its analogue, the nature/grace distinction (not dualism), the sacred/secular distinction is an important tool as we continue to learn how to navigate a post-Christian culture.
A significant part of the process of recovering and applying classical Reformed theology to our contemporary situation (sometimes called ressourcement, a French word which refers to getting back to original sources) is recovering the distinctions that we lost in the 19th and 20th centuries. There are a number of these, e.g., the archetypal/ectypal distinction, which, in Recovering the Reformed Confession, I called the categorical distinction; the distinction between law and gospel, which, in the classical period of Reformed theology (i.e., the 16th and 17th), was received as basic. Another lost distinction is that between the sacred and the secular. This is a distinction that our classical writers employed regularly but one that is regarded with suspicion today. In this discussion, sacred refers to that which is devoted to God. Think of the way Leviticus speaks of that which is dedicated to God or holy. Secular, in this context, refers to that which is common to Christians and pagans alike, which is not dedicated to God or holy in that sense. It does not mean “unclean” or defiled but simply not specially set apart. Think of the difference between the loaf of bread in your kitchen and the bread that has been consecrated for use in the Lord’s Supper. We often say during the administration of the Supper, “this sacred meal.” That there are secular meals is necessarily implied. Your family dinner is such a meal but it is not dirty or corrupt.
Recovering the Distinction Between Sacred and Secular
The traditional Christian (and Reformed) distinction is regarded with suspicion by some because it is unfamiliar. It is also, as a recent correspondent wrote to me, regarded by some as a Roman Catholic distinction. Some have been taught that the sacred belongs to God and the secular belongs to the Devil. That would be Manichaeism (i.e., the theology behind the Star Wars films). Others have been taught (directly or indirectly) by the followers of Abraham Kuyper that any distinction between the sacred and the secular somehow removes the sovereignty of God.
Neither of these was true in the classical period of Reformed theology and they are not true now. The Protestants saw the secular and the sacred as two distinct spheres over which and through which God exercises his sovereign providence.
Calvin used “secular” as a category without prejudice regularly. E.g. in Institutes 1.8.2, he contrasted the different styles between the human authors of sacred Scripture and “secular” writers. We see the same usage in 1.8.6. Calvin regularly wrote of secular judges, secular philosophers, secular work. E.g. in 4.7.22 he contrasted the properly sacred work of ministry with Gregory I’s complaint that he was forced to be too occupied with “secular affairs.” This way of thinking, speaking, and writing was universal among the magisterial Protestant Reformers and the Protestant orthodox.
We should not confuse the category secular with the use of “secular humanism” and “secularism” as pejoratives. Just as there is a difference between science and scientism so there is a proper distinction between things that are secular and a philosophy of secularism.
One way to think about the distinction between the sacred and the secular is to consider the restriction that the Apostle Paul placed on us in 1 Corinthians 10:14–21. The problem facing the Corinthian church was what to do about sharing meals with pagans.