This metaphysical statement comes from the healing textbook, Science and Health with Key to the Scriptures by Mary Baker Eddy. Chapter 14, titled Recapitulation, contains questions and answers on Christian Science.
Question. — What is the scientific statement of being?
Answer. — There is no life, truth, intelligence, nor substance in matter. All is infinite Mind and its infinite manifestation, for God is All-in-all. Spirit is immortal Truth; matter is mortal error. Spirit is the real and eternal; matter is the unreal and temporal. Spirit is God, and man is His image and likeness. Therefore man is not material; he is spiritual. SH 468:8-15
"Sickness has been combated for centuries by doctors using material remedies; but the question arises, Is there less sickness because of these practitioners?" Preface, Science and Health
"The physical healing of Christian Science results now, as in Jesus’ time, from the operation of divine Principle, before which sin and disease lose their reality in human consciousness and disappear as naturally and as necessarily as darkness gives place to light and sin to reformation. Now, as then, these mighty works are not supernatural, but supremely natural." Preface, Science and Health
"In Science man is the offspring of Spirit. The beautiful, good, and pure constitute his ancestry." Chapter on Marriage, Science and Health
"The prayer that reforms the sinner and heals the sick is an absolute faith that all things are possible to God, — a spiritual understanding of Him, an unselfed love." Chapter on Prayer, Science and Health