Let's set the stage, historically. By 200, there's been no ecumenical Church Councils since the Council of Jerusalem -- the Council of Nicea is still 125 years away. While the Books which would later become the Bible are widely circulated and seem to have generally been understood to be inspired Scripture, there are still some disagreements over which books are canonical, and even what "canonical" implies: namely, do we read a given Book in Church only if it's inspired? Or is it okay if it's uninspired, if it's still an accurate source of information about the Faith? (In modern terms, it would be like wondering if the Catechism should be one of the Readings in Mass). The first time we see the word "Trinity" used to describe God is in 181 A.D. The reality is there, but crafting a precise philosophical language to capture these realities takes time. In contrast to the kinks that the early Church was hammering out on everything from the Trinity to the Bible, their grasp of Eucharistic theology is almost shockingly clear. Even though philosophical terms like transubstantiation are far in the future, we're already seeing, by 200, terms like transmutation being used to describe what the words of consecration does to the bread and wine, and what the Eucharist does to our soul.
I. Didache (mid-first century)
The Didache is probably as old as the New Testament, and was in widespread use by the death of the Apostle John in 100 A.D. Unlike the Scriptures, the Didache isn't the work of a single author. Rather, it's something like an early Church catechism: outlining just the basics of Church practice. Chapter 9 is on the Eucharist, and after proscribing some beautiful and simple pre-Consecration prayers, it instructs: "But let no one eat or drink of your Eucharist, unless they have been baptized into the name of the Lord; for concerning this also the Lord has said, 'Give not that which is holy to the dogs.'" In the next chapter, there's a post-Communion prayer of thanksgiving, in which the Church prayed in part: "Thou, Master almighty, didst create all things for Thy name's sake; You gavest food and drink to men for enjoyment, that they might give thanks to Thee; but to us You didst freely give spiritual food and drink and life eternal through Thy Servant."
II. Ignatius of Antioch (c. 103-107 A.D.)
Ignatius dealt with the first Eucharistic controversy in the Church: the Gnostics. The Gnostics major heresy wasn't denying the Real Presence: rather, they denied that Jesus was fully God and fully Man. But as a result of this, Ignatius notes, they couldn't affirm the Eucharist, and thus, we can't commune with them. This is from Ignatius' letter to the Smyrnaeans, chapter 7:
They [the Gnostics] abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again. Those, therefore, who speak against this gift of God, incur death in the midst of their disputes. But it were better for them to treat it with respect, that they also might rise again. It is fitting, therefore, that ye should keep aloof from such persons, and not to speak of them either in private or in public, but to give heed to the prophets, and above all, to the Gospel, in which the passion [of Christ] has been revealed to us, and the resurrection has been fully proved. But avoid all divisions, as the beginning of evils.This same Ignatius, in his letter to the Ephesians, refers to the Eucharist as "the medicine of immortality."
III. Justin Martyr (150-155 A.D.)
Justin Martyr clearly shows that from the beginning, the Church held that not only was the Eucharist the Flesh and Blood of Christ, it also wasn't bread and wine after the consecration. Here's Chapter 66 of his First Apology:
And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.So once a certain prayer of His word is said, the bread and wine cease to be common bread and wine, and become spiritual Bread and Wine: namely, "the Flesh and Blood of that Jesus Who was made Flesh." The "prayer of His word" is the prayer of consecration, as Justin explains, quoting Christ at the Last Supper. What's translated there as "transmutation" is incredible. The actual phrase is "kata metabolen," and that metabolen is the root word of our word "metabolize." What Justin is actually saying is that by the Eucharist, our own body and blood is nourish and metabolized by Christ. Just as when we eat bread and drink wine, we turn the elements into our body through metabolism, when we eat the Eucharist, Christ metabolizes us (so to speak) into His Body. This is very much consistent with the view Scripture presents in places like 1 Corinthians 10:17.
IV.Irenaeus (180 A.D.)
Irenaeus was faced with a second Eucharistic heresy: this time, the heretics were claiming that spirit was good, but flesh and blood were evil, and that we were simply souls trapped in our evil bodies. Salvation, to these heretics, consisted of being liberated from flesh and blood. Irenaeus, in Book V, Chapter 2 of Against Heresies, used the example of the Eucharist to show that the material world isn't evil for three reasons: (1) material bread and wine, taken from the earth, become the Body and Blood of God; (2) the Eucharist is His physical Body and Blood, not some invisible spiritual "Body" [this point was assumed in Irenaeus' time, but is very much in controversy now]; and (3) through the Eucharist, we're promised that our bodies will, after death, be glorified in the same way the bread and wine are glorified. That part is fantastic. But there's an even clearer passage. In Book IV, Chapter 18, Irenaeus is dealing with the same heresy, and noting that these heretics still offer Mass. His argument is simple: they should either base their theology off of the Eucharist, like Catholics, or stop offering Mass:
Then, again, how can they 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.This passage is also helpful in that Irenaeus mentions that the bread ceases to be common bread at the point it becomes the Eucharist. It still has an earthly reality (we'd say "the accidents" of bread), but it's not bread anymore.
V. Tertullian (c. 203 A.D.)
I should mention that Tertullian, while brilliant on many points, isn't the most reliable Church Father. He became a Montanist later in life, and may have even died outside the Church. But we still can see quite clearly that he shares the same Eucharistic faith as those others we explored above. One of the issues Tertullian addressed in Chapter 19 of On Prayer was whether we should receive Communion on fast days (called "Station" days). He says yes, because it's the Lord's Body, and that's Who we're striving for. He also notes, as many of the above Fathers before him noted, that the Eucharist is truly a Sacrifice offered to the Father:
Similarly, too, touching the days of Stations, most think that they must not be present at the sacrificial prayers, on the ground that the Station must be dissolved by reception of the Lord's Body. Does, then, the Eucharist cancel a service devoted to God, or bind it more to God? Will not your Station be more solemn if you have withal stood at God's altar? When the Lord's Body has been received and reserved each point is secured, both the participation of the sacrifice and the discharge of duty.
As you can see, from the earliest days of the Church, we see the Church Fathers proclaiming unanimously that the Eucharist is truly the Body and Blood of Christ, and that the Eucharist truly saves. We also see them articulating that the bread and wine become the Eucharist at the prayer of consecration, and that once consecrated, the bread and wine cease to be common bread and wine. Christ doesn't just mingle amongst the elements. The elements cease to be, and become Christ instead, in what has been called transmutation, and would soon be called transelementation and eventually, transubstantiation. The rest of theology was understood through this prism of the Eucharist: if your views caused you to reject the Eucharist, you were out. The Christians would stop communing with you, and if you were a priest, you could no longer offer Mass in good faith. So this very Catholic understanding of the Eucharist served as a benchmark for determining orthodox Christianity from the very beginning.