“But our opinion is in accordance with the Eucharist, and the Eucharist in turn…”

But our opinion is in accordance with the Eucharist, and the Eucharist in turn establishes our opinion. For we offer to Him His own, announcing consistently the fellowship and union of the flesh and Spirit. For as the bread, which is produced from the earth, when it receives the invocation of God, is no longer common bread, but the Eucharist, consisting of two realities, earthly and heavenly; so also our bodies, when they receive the Eucharist, are no longer corruptible, having the hope of the resurrection to eternity.

Irenaeus · Against Heresies · ANF I, Against Heresies, Book IV, Chapter XVIII, 5

Irenaeus arguing against teachers who denied that the flesh can be saved, and reaching for the one thing his readers did every week: they bring bread and wine and believe they receive the body of the Lord, so they cannot also hold that flesh is beyond saving. It is the earliest clear statement of the principle that what a church prays is evidence for what it believes, and it is quoted in almost every later argument about the eucharist.