[An herbalist (for example) might recite a short prayer before collecting a plant. Before Christianity, his/her ancestors would have used a traditional folk invocation. After Christianity, the herbalist would replace the old formulae with something more suitable and more powerful (since Chrstianity was the “true” faith, and the old ways, while good, were fatally misguided).
By “Christianizing” old spells and rituals and incantations, people expressed their Christian faith, but at the same time maintained and protected the old ways—and themselves. Old way=evil. New way=good. However genuine your faith, you didn’t want to attract the wrong kind of attention from the Church. The Church, for its part, made ingratiating inroads in the other direction, consenting to consecrations of grounds and buildings, blessings of animals and property, and allowing the continuation of the old holy days and sacred grounds, with a new Christian understanding.]
“As Christianity grew in influence, a tension developed under the church and folk-medicine, since much in folk medicine was magical, or mystical, and had its foundation in pagan sources that were not compatible with Christian faith. Spells and incantations were used in conjunction with herbs and other remedies. Such spells had to be separated from the physical remedies, or replaced with Christian prayers or devotions. Similarly, the dependence upon the power of herbs or gems needed to be explained through Christianity.”
Pasted from <http://en.wikipedia.org/wiki/Medieval_medicine>
[We’d like to say that in the end the solution was to separate the spells from the physical remedies, but we’ve learned we can’t just rely on chemicals to heal us. In healing, set and setting are also important. Spells and incantations were part of a ritual which formalized (and standardized) the healer’s routine and manipulated the patient’s mental state toward good health.]