But, he who says that the old things have passed away, so that in Christ altar yields to altar, fire to prayers, animal victims to bread, blood to the chalice, does not know that the word “altare” is used quite often in the Law and the Prophets, and that an altar [altare] was first raised to God by Moses in the Tabernacle, while the word “ara” is also found in the writings of the Apostles, while the martyrs cry out under the altar [ara]. He says that the sword has yielded to fasting, forgetting that two-edged sword of both Testaments, with which the soldiers of the Gospel are armed. He says the fire has given place to prayers, as if prayers were not then offered in the temple, and fire is not now cast by Christ upon the world. He says that animal victims have been replaced by bread, as if he did not know that even then the loaves of proposition were placed upon the table of the Lord, and that now he partakes of the Body of the immaculate Lamb. He says that blood has given place to the chalice, not thinking that he now receives the Blood in the chalice. How much more truly and more appropriately could he say that the old things are passed away and are made new in Christ, so that altar yields to altar, sword to sword, fire to fire, bread to bread, victim to victim, blood to blood. Surely, we see by this that the carnal old things give place to spiritual newness. This, then, is what we have to understand – whether we dine on that changeable seventh day or whether some fast on that day – that the carnal sabbath has been transformed into the spiritual one, and that a true and eternal rest is looked for in the latter, while a merely physical rest is now despised in the former as a superstitious observance.Letter 36 (to Casulan), Chapter 10, Section 24 (translation from Fathers of the Church Series, Writings of St. Augustine, Volume 9, Letters, Volume 1(1-82), trans. Wilfrid Parsons, pp. 158-60)
The context of this quotation is a much longer letter from Augustine to Casulan regarding a pamphlet by an anonymous Roman author. The Roman author is trying to insist that Christians ought to fast on Saturday. Part of his rationale is premised on a consideration of the Old and New Testament administrations. As can be seen from the beginning of the discussion, he makes a variety of comparisons, which Augustine then proceeds to attempt to dismantle.
Frankly, Augustine's argument here is not exceptionally good. Nevertheless, the argument he employs shines some light on Augustine's view of the sacrament.
Notice how Augustine affirms that we receive "the Immaculate Lamb" in the Lord's Supper and "Blood" in the chalice. But this is in parallel to the "fire" that is cast by Christ upon the world (referring, doubtless, to the Holy Spirit) and the "two-edged sword" of the Bible. Finally, Augustine sums up his counter-point by taking the position that the physical/carnal has been replaced by the spiritual. Thus, a physical sword is replaced by the metaphorical sword of the Bible. The fire is replaced by the Holy Spirit, who is symbolized by fire. With prayers he notes that prayers continue and with altars, he points out that his opponent's linguistic point is incorrect.
When Augustine comes to bread replacing the animal sacrifices, Augustine provides an interesting double response. First, he points out that there was already bread (the shewbread) in the Old Testament. Next, he points out that we have an "animal sacrifice" in the form of the "Immaculate Lamb." Likewise, rather than animal blood, we have Christ's blood.
The way that this makes best sense within Augustine's argument is if Augustine understands "Lamb" and "blood" non-literally, but figuratively. A carnal sword with a spiritual sword, carnal fire with literal fire, carnal bread with spiritual bread, carnal victim with spiritual victim, carnal blood with spiritual blood, and (drumroll please!) therefore a carnal sabbath with a spiritual sabbath. In that spiritual sabbath we look forward to a true and eternal rest, not placing our hope in mere physical rest.
Although Augustine does not say that the Bible is a metaphorical sword or that the Holy Spirit is metaphorical fire, we can still figure that out from the context. Likewise, we can understand Augustine's metaphorical description of the bread and chalice. Augustine's punchline about the sabbath makes little sense if he means literal blood is replaced by literal blood, for example. Notice how in the conclusion of this argument he omits the prayers. Why? Because prayers are (for the purposes of his argument) the same, not spiritualized.
P.S. For those who want the original Latin, here it is:
Iste autem qui vetera transisse sic dicit, ut "in Christo cederet ara altari, gladius ieiunio, precibus ignis, pani pecus, poculo sanguis", nescit altaris nomen magis Legis et Prophetarum Litteris frequentatum, et altare Deo prius in tabernaculo, quod per Moysen factum est, collocatum; aram quoque in apostolicis Litteris inveniri, ubi Martyres clamant sub ara Dei. Dicit cessisse ieiunio gladium, non recordans illum quo milites evangelici armantur ex utroque Testamento, gladium bis acutum. Dicit cessisse precibus ignem, quasi non et tunc preces deferebantur in templum, et nunc a Christo ignis est missus in mundum. Dicit cessisse pani pecus, tanquamnesciens et tunc in Domini mensa panes propositionis poni solere, et nunc se de agni immaculati corpore partem sumere. Dicit cessisse poculo sanguinem, non cogitans etiam nunc se accipere in poculo sanguinem. Quanto ergo melius et congruentius vetera transisse, et nova in Christo facta esse sic diceret, ut cederet altare altari, gladius gladio, ignis igni, panis pani, pecus pecori, sanguis sanguini. Videmus quippe in his omnibus carnalem vetustatem spiritali cedere novitati. Sic ergo intellegendum est, sive in isto die volubili septimo prandeatur, sive a quibusdam etiam ieiunetur, tamen sabbato spiritali sabbatum carnale cessisse; quando in isto sempiterna et vera requies cuncupiscitur, in illo vacatio temporalis iam superstitiosa contemnitur.