Catholic Understanding of the Eucharist
"That Bread which you see on the altar, having been sanctified by the word of God IS THE BODY OF CHRIST. That chalice, or rather, what is in that chalice, having been sanctified by the word of God, IS THE BLOOD OF CHRIST. Through that bread and wine the Lord Christ willed to commend HIS BODY AND BLOOD, WHICH HE POURED OUT FOR US UNTO THE FORGIVENESS OF SINS." (St. Augustine, Sermons 227)
"The Lord Jesus wanted those whose eyes were held lest they should recognize him, to recognize Him in the breaking of the bread [Luke 24:16,30-35]. The faithful know what I am saying. They know Christ in the breaking of the bread. For not all bread, but only that which receives the blessing of Christ, BECOMES CHRIST'S BODY." (St. Augustine, Sermons 234:2)
"What you see is the bread and the chalice; that is what your own eyes report to you. But what your faith obliges you to accept is that THE BREAD IS THE BODY OF CHRIST AND THE CHALICE [WINE] THE BLOOD OF CHRIST." (St. Augustine, Sermons 272)
"How this ['And he was carried in his own hands'] should be understood literally of David, we cannot discover; but we can discover how it is meant of Christ. FOR CHRIST WAS CARRIED IN HIS OWN HANDS, WHEN, REFERRING TO HIS OWN BODY, HE SAID: 'THIS IS MY BODY.' FOR HE CARRIED THAT BODY IN HIS HANDS." (St. Augustine, Psalms 33:1:10)
The Priest and the Mass
"Christ is both the Priest, OFFERING Himself, and Himself the Victim. He willed that the SACRAMENTAL SIGN of this should be the daily Sacrifice of the Church, who, since the Church is His body and He the Head, learns to OFFER herself through Him." (St. Augustine, City of God 10:20)
"By those sacrifices of the Old Law, this one Sacrifice is signified, in which there is a true remission of sins; but not only is no one forbidden to take as food the Blood of this Sacrifice, rather, all who wish to possess life are exhorted to drink thereof." (St. Augustine, Questions on the Heptateuch 3:57)
"...I turn to Christ, because it is He whom I seek here; and I discover how the earth is adored without impiety, how without impiety the footstool of His feet is adored. For He received earth from earth; because flesh is from the earth, and He took flesh from the flesh of Mary. He walked here in the same flesh, AND GAVE US THE SAME FLESH TO BE EATEN UNTO SALVATION. BUT NO ONE EATS THAT FLESH UNLESS FIRST HE ADORES IT; and thus it is discovered how such a footstool of the Lord's feet is adored; AND NOT ONLY DO WE NOT SIN BY ADORING, WE DO SIN BY NOT ADORING." (St. Augustine, Psalms 98:9)
The Mass, The Eucharist and Purgatory:
"But by the prayers of the Holy Church, and by the SALVIFIC SACRIFICE, and by the alms which are given for their spirits, there is no doubt thatthe dead are aided that the Lord might deal more mercifully with them than their sins would deserve. FOR THE WHOLE CHURCH OBSERVES THIS PRACTICE WHICH WAS HANDED DOWN BY THE FATHERS that it prays for those who have died in the communion of the Body and Blood of Christ, when they are commemorated in their own place in the Sacrifice itself [part of the Mass mentions the Saints who have "gone before us"]; and the Sacrifice is OFFERED also in memory of them, on their behalf. If, the works of mercy are celebrated for the sake of those who are being remembered, who would hesitate to recommend them, on whose behalf prayers to God are not offered in vain? It is not at all to be doubted that such prayers are of profit to the dead; but for such of them as lived before their death in a way that makes it possible for these things to be useful to them after death." (St. Augustine, Sermons 172:2)
"There is an ecclesiastical discipline, as the faithful know, when the names of the martyrs are read aloud in that place at the altar of God, where prayer is not offered for them. Prayer, however, is offered for other dead who are remembered. It is wrong to pray for a martyr, to whose prayers we ought ourselves be commended" (Sermons 159:1 [A.D. 411]).
"Temporal punishments are suffered by some in this life only, by some after death, by 'some both here and hereafter, but all of them before that last and strictest judgment. But not all who suffer temporal punishments after death will come to eternal punishments, which are to follow after that judgment" (The City of God 21:13 [A.D. 419]).
"The prayer either of the Church herself or of pious individuals is heard on behalf of certain of the dead, but it is heard for those who, having been regenerated in Christ, did not for the rest of their life in the body do such wickedness that they might be judged unworthy of such mercy [as prayer], nor who yet lived so well that it might be supposed they have no need of such mercy [as prayer]" (ibid., 21:24:2).
"That there should be some fire even after this life is not incredible, and it can be inquired into and either be discovered or left hidden whether some of the faithful may be saved, some more slowly and some more quickly in the greater or lesser degree in which they loved the good things that perish, through a certain PURGATORIAL FIRE" (Handbook on Faith, Hope, and Charity 18:69 [A.D. 421]).
"The time which interposes between the death of a man and the final resurrection holds souls in hidden retreats, accordingly as each is deserving of rest or of hardship, in view of what it merited when it was living in the flesh. Nor can it be denied that the souls of the dead find relief through the piety of their friends and relatives who are still alive, when the Sacrifice of the Mediator [Mass] is offered for them, or when alms are given in the Church. But these things are of profit to those who, when they were alive, merited that they might afterward be able to be helped by these things. There is a certain manner of living, neither so good that there is no need of these helps after death, nor yet so wicked that these helps are of no avail after death" (ibid., 29:109).
source : http://www.americancatholictruthsociety.com/articles/augustinecatholic.htm