Distinct, unchanged natures in Christ and in the Sacrament...

In addition to Christ's and St. Paul's expressions (the bread in the Supper is the body of Christ or the communion of the body of Christ), the following forms are also used:  under the bread, with the bread, in the bread.  With these words, the papistic transubstantiation may be rejected and the sacramental union of the bread's unchanged essence and Christ's body may be shown.  In the same way, the expression "the Word became flesh" (John 1:14) is repeated and explained by the equivalent expressions, "The Word...dwelt among us" (John 1:14); likewise, "in Him the whole fullness of the deity dwells bodily" (Colossians 2:9); likewise, "God was with Him" (Acts 10:38); likewise, "in Christ God was" (2 Corinthians 5:19), and the like.  These show that the divine natures, unchanged, are personally united.  Many eminent ancient teachers, such as Justin, Cyprian, Augustine, Leo, Gelasius, Chrysostom, and others, use this comparison about the words of Christ's testament, "This is My body."  Just as in Christ two distinct, unchanged natures are inseparably united--the natural bread and Christ's true natural body--are present together here on earth in the appointed administration of the Sacrament.
~BOC, FSD, VII, 35-37