Some Christian theologians embrace a holistic combination of Christian theology with some ideas of (ontological) materialism, a belief that matter is a fundamental substance of the world and that mental phenomena result from matter. Historical background Throughout history, Christian thought has struggled with the ideas of flesh, world, and spirit, and their interplay in each person's salvation. As Cardinal Joseph Ratzinger (Pope Benedict XVI) said in What It Means To Be a Christian (2006), "Christian theology... in the course of time turned the kingdom of God into a kingdom of heaven that is beyond this mortal life; the well-being of men became a salvation of souls, which again comes to pass beyond this life, after death." This tendency of spiritualization, Ratzinger said, is not the message of Jesus Christ. "For what is sublime in this message," he stated, "is precisely that the Lord was talking not just about another life, not just about men's souls, but was addressing the body, the whole man, in his embodied form, with his involvement in history and society; that he promised the kingdom of God to the man who lives bodily with other men in this history." Christian materialism is a widely discussed position in contemporary analytic philosophy of religion, defended by figures such as Peter van Inwagen and Trenton Merricks. Pope John Paul II's Theology of the Body lectures asserted that "The body, and it alone, is capable of making visible ... the spiritual and divine." Christopher West, in reviewing the words of Pope John Paul II, asserted Archbishop William Temple has remarked that Christianity is "the most avowedly materialist of all the great religions." Rejection of 'materialism' According to Constantin Gutberlet writing in Catholic Encyclopedia (1911), "materialism", defined as "a philosophical system which regards matter as the only reality in the world…denies the existence of God and the soul." In this view, materialism could be perceived incompatible with world religions that ascribe existence to immaterial objects. Materialism may be conflated with atheism; according to Friedrich A. Lange (1892), "Diderot has not always in the Encyclopædia expressed his own individual opinion, but it is just as true that at its commencement he had not yet got as far as Atheism and Materialism." Mary Baker Eddy, the founder of the Christian Science movement, denied the existence of matter on the basis of the allness of Mind (which she regarded as a synonym for God).