Consubstantiation is a philosophical theory that, like the competing theory of transubstantiation, attempts to describe the nature of the Christian Eucharist in concrete metaphysical terms. It holds that during the sacrament the fundamental substance of the body and blood of Christ are present alongside the substance of the bread and wine, which remain present. Transubstantiation differs from consubstantiation in that it postulates that, through consecration by the priest, one set of substances (bread and wine) is exchanged for another (the Body and Blood of Christ) or that, according to some, the reality of the bread and wine become the reality of the body and blood of Christ. The substance of the bread and wine do not remain, but their accidents (superficial properties like appearance and taste) remain.
Consubstantiation is commonly—though erroneously—associated with the teachings of Martin Luther and Philipp Melanchthon. Lutheran teachings reject any attempt to explain philosophically the means by which Christ is present in the Eucharist. Luther did teach that the body and blood of Christ are present "in, with, and under the forms" of bread and wine, and present-day Lutherans hold to this statement while disagreeing about its exact meaning. Some Lutherans do use the term "consubstantiation" to refer to this belief, but the theology intended is not the same as the philosophical theory described above. Luther illustrated his belief about the Eucharist "by the analogy of the iron put into the fire whereby both fire and iron are united in the red-hot iron and yet each continues unchanged," ^ 1^ a concept which he called sacramental union. Consubstantiation is affirmed by a minority of Christians, including some Eastern Orthodox churches.
- Against the Heavenly Prophets (1525) and Confession Concerning Christ's Supper (1528) as quoted in The Oxford Dictionary of the Christian Church, F.L. Cross, Ed., London: Oxford, 1958, p. 337.