Protestants accuse Catholics of "adding" books to the Bible, while Catholics retort that Protestants have "booted out" part of Scripture. Catholics are able to offer very solid and reasonable arguments in defense of the scriptural status of the deuterocanonical books. These can be summarized as follows:
1) They were included in the Septuagint (the Greek translation of the Old Testament from the third century B.C.), which was the "Bible" of the Apostles. They usually quoted the Old Testament scriptures (in the text of the New Testament) from the Septuagint.
2) Almost all of the Church Fathers regarded the Septuagint as the standard form of the Old Testament. The deuterocanonical books were in no way differentiated from the other books in the Septuagint, and were generally regarded as canonical. St. Augustine thought the Septuagint was apostolically-sanctioned and inspired, and this was the consensus in the early Church.
3) Many Church Fathers (such as St. Irenaeus, St. Cyprian, Tertullian) cite these books as Scripture without distinction. Others, mostly from the east (for example, St. Athanasius, St. Cyril of Jerusalem, St. Gregory Nazianzus) recognized some distinction but nevertheless still customarily cited the deuterocanonical books as Scripture. St. Jerome, who translated the Hebrew Bible into Latin (the Vulgate, early fifth century), was an exception to the rule (the Church has never held that individual Fathers are infallible).
4) The Church Councils at Hippo (393) and Carthage (397, 419), influenced heavily by St. Augustine, listed the deuterocanonical books as Scripture, which was simply an endorsement of what had become the general consensus of the Church in the west and most of the east. Thus, the Council of Trent merely reiterated in stronger terms what had already been decided eleven and a half centuries earlier, and which had never been seriously challenged until the onset of Protestantism.
5) Since these Councils also finalized the 66 canonical books which all Christians accept, it is quite arbitrary for Protestants to selectively delete seven books from this authoritative Canon. This is all the more curious when the complicated, controversial history of the New Testament Canon is understood.
6) Pope Innocent I concurred with and sanctioned the canonical ruling of the above Councils (Letter to Exsuperius, Bishop of Toulouse) in 405.
7) The earliest Greek manuscripts of the Old Testament, such as Codex Sinaiticus (fourth century), and Codex Alexandrinus (c.450) include all of the deuterocanonical books mixed in with the others and not separated.
8) The practice of collecting these books into a separate unit dates back no further than 1520 (in other words, it was a novel innovation of Protestantism). This is admitted by, for example, the Protestant New English Bible (Oxford University Press, 1976), in its "Introduction to the Apocrypha," (p.iii).
9) Protestantism, following Martin Luther, removed the deuterocanonical books from their Bibles due to their clear teaching of doctrines which had been recently repudiated by Protestants, such as prayers for the dead (Tobit 12:12, 2 Maccabees 12:39-45 ff.; cf. 1 Corinthians 15:29), intercession of dead saints (2 Maccabees 15:14; cf. Revelation 6:9-10), and intermediary intercession of angels (Tobit 12:12,15; cf. Revelation 5:8, 8:3-4). We know this from plain statements of Luther and other Reformers.
10) Luther was not content even to let the matter rest there, and proceeded to cast doubt on many other books of the Bible which are accepted as canonical by all Protestants. He considered Job and Jonah mere fables, and Ecclesiastes incoherent and incomplete. He wished that Esther (along with 2 Maccabees) "did not exist," and wanted to "toss it into the Elbe" river.
11) The New Testament fared scarcely better under Luther's gaze. He rejected from the New Testament Canon ("chief books") Hebrews, James ("epistle of straw"), Jude and Revelation, and placed them at the end of his translation, as a New Testament "Apocrypha." He regarded them as non-apostolic. Of the book of Revelation he said, "Christ is not taught or known in it." These opinions are found in Luther's Prefaces to biblical books, in his German translation of 1522.