Central to this is the understanding that when the Church offers the Eucharist, it offers a true Sacrifice, something it received from the apostles who received it from Christ. Like Melchizedek, we offer bread and wine, the first-fruits of created things, and God transforms them into the Body and Blood of Christ who was offered once for sin and who was resurrected, the first-fruits of the New Creation, in which we are made partakers. In that way, we participate in and proclaim the One Sacrifice of Christ and also His Resurrection from the dead. From Ch. 17 of Book IV of "Against Heresies":
Again, giving directions to His disciples to offer to God the first-fruits of His own, created things (not as if He stood in need of them, but that they might be themselves neither unfruitful nor ungrateful), He took that created thing, bread, and gave thanks, and said, This is My body. And the cup likewise, which is part of that creation to which we belong, He confessed to be His blood, and taught the new oblation of the new covenant; which the Church receiving from the apostles, offers to God throughout all the world...Ch. 18):
.... But how can [these Gnostics] be consistent with themselves, [when they say] that the bread over which thanks have been given is the body of their Lord, and the cup His blood, if they do not call Himself the Son of the Creator of the world, that is, His Word, through whom the wood fructifies, and the fountains gush forth, and the earth gives first the blade, then the ear, then the full grain in the ear.Bingo. Why are the Gnostics wrong? Because the Eucharist confirms their inconsistency and their incoherency. Let them therefore either alter their opinion, or cease from offering the Eucharist. St. Irenaeus of Lyons, pray for us!
Then, again, how can [the Gnostics] say that the flesh, which is nourished with the body of the Lord and with His blood, goes to corruption, and does not partake of life? Let them, therefore, either alter their opinion, or cease from offering the things just mentioned. 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.