In the field of Reformed apologetics we sometimes speak about the antithesis. The antithesis is a theological principle that is meant to describe the difference between believers and unbelievers. There are many ways that we could describe that difference, but we must at the very least describe that difference covenantally. That is, it describes a distinction between those who are in Adam and under the terms of the covenant of works and those who are in Christ—those who Christ has redeemed and brought into the covenant of grace. There is a covenantal distinction between these groups.
But we should also recognize that not only is this antithesis covenantal, it is absolute in the sense that there are no other categories of human beings. There is no one who is in some third group. We have those in Adam and those in Christ; there are none others. There is no middle ground. There is no neutrality. There are the two groups of people: those who are still children of wrath and those who have been redeemed by God’s grace and brought into the covenant of grace by the Holy Spirit applying the life death and resurrection of Christ unto them.
The antithesis is covenant and absolute. The antithesis is also ethical. We should not in other words understand this drastic divide between believers and unbelievers as an ontological difference. We should not understand the antithesis as meaning believers are human beings while unbelievers are something other than human beings. Certainly, we recognize that the Holy Spirit works in the hearts and minds of believers. He enlightens their minds and renews their wills so that they would embrace Jesus Christ as he is offered to us in the gospel. We also know that all those born in sin have had their minds darkened. Their hearts are turned against God, and they serve in active rebellion against God, seeking to suppress the truth in unrighteousness. Those are legitimate and significant differences—not merely a figure of speech or words used to describe something that isn’t actually there. Nevertheless, all humans are indeed still humans after the fall into sin.
Still, once Adam fell into sin there’s a great divide, an absolute ethical antithesis that is covenantally determined between those who are in Adam and those who are in Christ. In other words, there is a distinction between those who are children of wrath and those who have been redeemed, are being sanctified, and will arrive at the final day to meet their savior. Christ will bring them as fully sanctified and holy people, fully redeemed, consummate, and glorified people into the New Heavens and the New Earth.