Another terrific essay on one of my favorite websites, Called to Communion. Authored to Tim A. Troutman, it begins this way:
This article is intended to be a resource showing the support for the doctrine of Transubstantiation in the Church fathers, and not a robust defense of the doctrine as defined by the Council of Trent. The Church fathers did not believe in a mere spiritual presence of Christ alongside or in the elements (bread and wine). This can be shown by three different types of patristic statements. The first and most explicit type is a statement that directly affirms a change in the elements. The second type is a simple identification of the consecrated species with the Body and Blood of Jesus Christ. Because unconsecrated bread is not called the Body, and consecratedis called the Body, this directly implies a belief that a supernatural change has taken place at the point of consecration. The third and final type is a statement which attributes or demands extraordinary reverence for the consecrated species itself, and not merely the solemnity of communion in this sacrament.
Read the whole thing here.