The statement "Catholics added books to the Bible at the Council of Trent" is too simple to describe the history accurately.
The books Catholics call Deuterocanonical were known, copied and used by Christians long before the sixteenth-century Reformation.
At the same time, the history of the Old Testament canon was not completely uniform in every ancient Christian community.
Both facts matter.
Why are there 73 books in a Catholic Bible and 66 in most Protestant Bibles?
The difference concerns seven Old Testament books --- Tobit, Judith, Wisdom, Sirach, Baruch, 1 Maccabees and 2 Maccabees --- plus portions of Esther and Daniel.
Catholics receive these as Scripture.
Most Protestant traditions place them outside the canon, historically often calling them the Apocrypha.
Were these books known before Trent?
Yes.
They occur within the broader textual tradition of the Greek Septuagint used widely by early Christians, and ancient Christian canonical lists and manuscripts show that these books were not a sixteenth-century Catholic invention.
The Council of Trent in the sixteenth century gave a definitive Catholic response during Reformation controversy; it did not discover these books for the first time.
Was there ancient disagreement?
Yes.
Ancient writers did not all describe the Old Testament canon identically.
Jerome, for example, distinguished certain books in ways important to later discussions.
A strong Catholic historical argument should acknowledge those complexities rather than pretend every Christian canon list was identical from the first century onward.
The better question
Instead of asking only:
Who added or removed books?
ask:
Which Old Testament books were received in Christian worship and tradition, how did that reception develop, and on what authority should the canon be determined?
That is the question worth investigating.
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