Gordon Spykman hits the nail on the head:

In dualisms the divine norm is always either kept at a distance, a step removed from everyday living (‘upstairs’), or it is identified with some aspect of life (‘downstairs’), or it takes the form of a dual normativity which wavers dialectically between the two. Dualism is a deceptive attempt to reject life in the world (in part) while at the same time also accepting it (in part). It tends to break rather than to absorb the tension between the ‘already’ and the ‘not yet’ in the biblical vision of the coming kingdom: some parts of life are viewed as ‘already’ under the rule of Christ, others ‘not yet.’ Christian faith is often related only extrinsically to scholarship. All such dualisms make it impossible to do justice to the biblical message of creation/fall/redemption as holistic realities. For they disrupt the unity of the creation order. They legitimize the reality of sin in on or another realm of life. They limit the cosmic impact of the biblical message of redemption. They confine Christian witness to only limited sectors of life (Gordon Spykman, Reformational Theology, p. 68, emphasis mine).

I find this to be a much more appealing vision of Christ’s redemptive work than those which would exclude certain sectors of life from the effects of redemption and instead would call them “secular” or “neutral.” The question, as Spykman notes 2 pages prior, is not whether a sphere of life is sacred or secular, but whether those in it are obedient to Christ or to the powers of sin, death, and evil. To quote him again,

Christian causes stand in principle behind the thesis that Christ is Lord of all. So-called ‘neutral’ organizations and institutions, which are in reality humanist and secular, are in principle ‘antithetical’ and ‘separate.’ For they fail to stand on the side of the biblical thesis. They have in effect separated themselves from the renewed order of reality, namely, that ‘God is in Christ reconciling all things to himself’ (2 Corinthians 5:19). So now the basic question we all face is this: Are we for Christ or for some anti-Christ? This thetical/antithetical decision is radical and all-embracing in its impact (Ibid., 66, emphasis mine).