This is, however, very strange since St Augustine was Catholic.
And he wasn't just any Catholic, he was the Catholic Bishop of Hippo. And as such, he believed and taught the faith of the Catholic Church of his day (which, unsurprisingly, has been preserved to our own day 1600 years later). Setting aside the much more complex question of what exactly Augustine believed regarding justification, below is just a small sample of quotes from his writings that confirm that he held many distinctively Catholic positions.
The Catholic Church is the true Church
"[T]here are many other things which most justly keep me in her bosom. The consent of peoples and nations keeps me in the Church; so does her authority, inaugurated by miracles, nourished by hope, enlarged by love, established by age. The succession of priests keeps me, beginning from the very seat of the Apostle Peter, to whom the Lord, after His resurrection, gave it in charge to feed His sheep, down to the present episcopate. And so, lastly, does the name itself of Catholic, which, not without reason, amid so many heresies, the Church has thus retained; so that, though all heretics wish to be called Catholics, yet when a stranger asks where the Catholic Church meets, no heretic will venture to point to his own chapel or house. Such then in number and importance are the precious ties belonging to the Christian name which keep a believer in the Catholic Church, as it is right they should..." (Against the Fundamental Epistle of Manichaeus, 4)
Tradition as an authority
""The apostles," indeed, "gave no injunctions on the point;" but the custom [of not re-baptizing converts] may be supposed to have had its origin in apostolic tradition, just as there are many things which are observed by the whole Church, and therefore are fairly held to have been enjoined by the apostles, which yet are not mentioned in their writings." (On Baptist, Against the Donatists, 23.31)
"As to those other things which we hold on the authority, not of Scripture, but of tradition, and which are observed throughout the whole world, it may be understood that they are held as approved and instituted either by the apostles themselves, or by plenary Councils, whose authority in the Church is most useful..." (Letter 54.1)
"Now the whole canon of Scripture on which we say this judgment is to be exercised, is contained in the following books:— Five books of Moses, that is, Genesis, Exodus, Leviticus, Numbers, Deuteronomy; one book of Joshua the son of Nun; one of Judges; one short book called Ruth, which seems rather to belong to the beginning of Kings; next, four books of Kings [1&2 Samuel and 1&2 Kings], and two of Chronicles— these last not following one another, but running parallel, so to speak, and going over the same ground. The books now mentioned are history, which contains a connected narrative of the times, and follows the order of the events. There are other books which seem to follow no regular order, and are connected neither with the order of the preceding books nor with one another, such as Job, and Tobias, and Esther, and Judith, and the two books of Maccabees, and the two of Ezra [Ezra & Nehemiah], which last look more like a sequel to the continuous regular history which terminates with the books of Kings and Chronicles. Next are the Prophets, in which there is one book of the Psalms of David; and three books of Solomon, viz., Proverbs, Song of Songs, and Ecclesiastes. For two books, one called Wisdom and the other Ecclesiasticus, are ascribed to Solomon from a certain resemblance of style, but the most likely opinion is that they were written by Jesus the son of Sirach. Still they are to be reckoned among the prophetical books, since they have attained recognition as being authoritative. The remainder are the books which are strictly called the Prophets: twelve separate books of the prophets which are connected with one another, and having never been disjoined, are reckoned as one book; the names of these prophets are as follows:— Hosea, Joel, Amos, Obadiah, Jonah,Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi; then there are the four greater prophets, Isaiah, Jeremiah [which, at that time, included Lamentations and Baruch], Daniel, Ezekiel. The authority of the Old Testament is contained within the limits of these forty-four books [plus 2 for Lamentations and Baruch] [He then goes on to list the New Testament books]." (On Christian Doctrine II.8.13)
“This is the meaning of the great sacrament of baptism, which is celebrated among us: all who attain to this grace die thereby to sin—as he himself [Jesus] is said to have died to sin because he died in the flesh (that is, ‘in the likeness of sin’)—and they are thereby alive by being reborn in the baptismal font, just as he rose again from the sepulcher. This is the case no matter what the age of the body. For whether it be a newborn infant or a decrepit old man—since no one should be barred from baptism—just so, there is no one who does not die to sin in baptism. Infants die to original sin only; adults, to all those sins which they have added, through their evil living, to the burden they brought with them at birth” (Handbook on Faith, Hope, and Love 13)
Real Presence in the Eucharist
"I promised you [new Christians], who have now been baptized, a sermon in which I would explain the sacrament of the Lord's Table...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. [...] 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 that the bread is the body of Christ and the chalice is the blood of Christ. This has been said very briefly, which may perhaps be sufficient for faith; yet faith does not desire instruction..." (Sermons 227)
Venial/Mortal Sin Distinction and Need of Penance for Mortal Sins
“When you shall have been baptized, keep to a good life in the commandments of God so that you may preserve your baptism to the very end. I do not tell you that you will live here without sin, but they are venial sins which this life is never without. Baptism was instituted for all sins. For light sins, without which we cannot live, prayer was instituted. . . . But do not commit those sins on account of which you would have to be separated from the body of Christ. Perish the thought! For those whom you see doing penance have committed crimes, either adultery or some other enormities. That is why they are doing penance. If their sins were light, daily prayer would suffice to blot them out. . . . In the Church, therefore, there are three ways in which sins are forgiven: in baptisms, in prayer, and in the greater humility of penance” (Sermon to Catechumens on the Creed 7.15, 8.16)
“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.” (Handbook on Faith, Hope, and Charity, 29.109)
Prayers for the Dead and Asking for the prayers of the saints
"This it was that the blessed martyrs did in their burning love; and if we celebrate their memories in no mere empty form, and, in the banquet whereat they themselves were filled to the full, approach the table of the Lord, we must, as they did, be also ourselves making similar preparations. For on these very grounds we do not commemorate them at that table in the same way, as we do others who now rest in peace, as that we should also pray for them, but rather that they should do so for us, that we may cleave to their footsteps; because they have actually attained that fullness of love, than which, our Lord has told us, there cannot be a greater." (Lectures on the Gospel of John 84.1)
Perpetual Virginity of Mary
“It was not the visible sun, but its invisible Creator who consecrated this day for us, when the Virgin Mother, fertile of womb and integral in her virginity, brought him forth, made visible for us, by whom, when he was invisible, she too was created. A Virgin conceiving, a Virgin bearing, a Virgin pregnant, a Virgin bringing forth, a Virgin perpetual. Why do you wonder at this, O man?” (Sermons 186.1)
Sinlessness of Mary (considered)“Having excepted the holy Virgin Mary, concerning whom, on account of the honor of the Lord, I wish to have absolutely no question when treating of sins—for how do we know what abundance of grace for the total overcoming of sin was conferred upon her, who merited to conceive and bear him in whom there was no sin?—so, I say, with the exception of the Virgin, if we could have gathered together all those holy men and women, when they were living here, and had asked them whether they were without sin, what do we suppose would have been their answer?” (Nature and Grace 36.42)
Mary is the mother of all Christians
“That one woman is both mother and virgin, not in spirit only but even in body. In spirit she ismother, not of our head, who is our Savior himself—of whom all, even she herself, are rightly called children of the bridegroom—but plainly she is the mother of us who are his members, because by love she has cooperated so that the faithful, who are the members of that head, might be born in the Church. In body, indeed, she is the Mother of that very head” (Holy Virginity 6.6)
