Dualism is the dividing of life into sacred and secular, spiritual and non-spiritual, religious and non-religious.  It divides allegiances.  Dietrich Bonnhoffer comments:

The division of total reality into a sacred and a profane sphere, a Christian and a secular sphere, creates the possibility of existence in a single one of these spheres, a spiritual existence which has no part in secular existence, and a secular existence which can claim autonomy for itself and can exercise this right of autonomy in its dealings with the spiritual sphere. ... (‘Christ, the church and the world’ in Ray S Anderson (ed.) Theological Foundations for Ministry T&T Clark p. 540.)

The net result is schizoid Christians.

Dualism is rooted in the fall: structure and direction become confused.  In part its influence in contemporary Christianity can be traced back to Greek philosophy, and to Plato (427-347 BC) in particular; it is alien to the scriptures.  Putting it rather crudely reality, according to Plato, comprised of two separate realms, the material and the spiritual.  The spiritual was thought to be superior to the material.  Plato's ideas and thinking had a tremendous impact on the early church fathers.  Justin Martyr (c. 100-165) even described Plato as a Christian before Christ.  Many Christians are unconsciously influenced by these platonic ideas, and consequently so-called spiritual activities such as worship prayer and evangelism are regarded as more important than education, science, politics or caring for the earth.

This separation of life into sacred and secular, or spiritual and material, is absent from the Old and New Testaments.  Indeed the biblical evidence is very much to the contrary: all of life is spiritual.  Six times in Genesis 1 God affirmed the goodness of the material realm.  Elsewhere, we find that God loved the world so much that he sent his only son to redeem it;  Jesus, God's son, became human and his resurrection body was a physical one - he was not a disembodied spirit (cf 1 Jn 1:1ff), and it is the material realm that will be redeemed and restored at the consummation of all things.

There is no dualism in scripture.  Education, science, technology, mathematics, philosophy, politics, business, ... are just as much "spiritual" activities as theology,prayer, praise and evangelism.  The one-time Dutch primeminister and theologian, Abraham Kuyper summed it up:  "There is not one inch of secular life of which Christ does not say 'It belongs to me'".  It is failing to grasp the full impact of this truth that Christians have come to regard somethings as more Christian than others, and this is reflected in how they choose to live their lives and occupy their time, and indeed where their commitment lies.  It is dualism that regards some areas of life as being more important for Christian involvement than others.  Christianity belongs just as much in the science lab and the shopping mall as it does in school assemblies and church services.

Dualism is a result of the fall, it arises out of ontologising the antithesis – structure and direction become confused. 

In dualism there is a higher realm and a lower realm. The higher realm is thought to be normative and is needed to redeem or save the corresponding lower realm.  The lower realm is considered inherently evil, or can only partly be redeemed.  These higher and lower realms could be:

Grace        Sacred    Church       Spiritual    Faith       
Nature       Secular    World        Secular    Reason

The list could go on. 

But ‘All of life is religion’ – as H Evan Runner declared.  Reformational Christianity rejects any dualism; it is a holistic and integrated Christianity.  The fall affects all of life, but so too does redemption.  It is redemption that we will look at next.

Further reading
Al Wolters Creation Redeemed (IVP, 1985) Ch 3
B J van der Walt Transformed by the Renewing of Your Mind (IRS, Potchefstroom, 2001) ch 1.
Gordon Spykman Reformational Theology (Eerdmans, Grand Rapids, 1992) pp 66-68