Christian Science is usually encountered as a position about medicine, and the position about medicine is a consequence of something prior. The prior claim is about what exists. Mary Baker Eddy held that God is Spirit, wholly good, and the only genuine cause, from which she drew the conclusion that whatever is real must be spiritual and good. Matter, on that reasoning, cannot be a second kind of stuff that God also made, and evil, disease and death cannot be things God caused; since nothing exists apart from God, they are not things at all, but errors of belief held by what she called mortal mind. The claim is deductive rather than evidential, and Eddy presented it that way, as a science in the older sense of a demonstrable body of knowledge rather than as a set of pious hopes.
Healing follows as the demonstration. If a diseased body is a false belief rather than a damaged mechanism, then it yields to correction of the belief, which is what Christian Science prayer sets out to do, not by asking God to intervene in matter but by understanding more clearly the spiritual facts that were always the case.
Eddy set the whole out in Science and Health with Key to the Scriptures, published in 1875 and revised by her repeatedly for the rest of her life, and she paired the book with the Bible as the church's authority. She was insistent that the teaching had been revealed to her rather than derived from any teacher, a point on which she had reason to be emphatic, since her early association with the mental healer Phineas Quimby was used against her for decades.