Why does the Catholic Bible contain 73 books while the Protestant Bible has only 66? Also, with both claiming to have the complete word of God, which one should be believed and why?

It was the Catholic Bishops together with sister Orthodox who canonized the New Testament & decided which Books of the Old Testament to include. The Orthodox Church has 3 more Books than Catholic.

The Old Testament had 46 Books & the New Testament 27.

One of the essential criterion in respect of the Old Testament was the Books the Savior Jesus Christ quoted from. So no matter how obscure Luther 1300 years later was to call the Books he didn’t approve of, if the Savior quoted from them in His public ministry, they are regarded by CC as authoritative.

Luther threw out 7 Books as he said they were obscure. But if the Lord quoted from them then obviously He didn’t think so.

Also, Luther dispensed with the books which support Catholic doctrine (where the doctrine came from) so that having dispensed with the books could then say ‘Catholics believe things not in the Bible!’

Then Luther added odd single words to change the meaning of a text. E.g. ‘We are saved by faith’, added the word ‘alone’ so the text now reads ‘we are saved by faith’ alone’. Whereas numerous books of New Testament say ‘once saved, justify Faith by good works.’ The word ‘alone’ is a good cop out clause: hey we are saved by faith’ alone’ so we don’t have to be nice to people & give alms to the poor because the text says we are saved by faith’ alone’ & we binned all the Books which say we got to justify our Faith by works.

You will hear some folk actually condemning being charitable because the Protestant Bible doesn’t say we have to & condemning Catholics & Orthodox because their bibles still got the original Books in situ that says we DO have to justify our Faith by good works

That is one of the reasons why the Catholic Church has built, paid for & runs hundreds of hospitals world wide (one recently been in the news in Syria as Gov’t forces keep bombing it because some patients are terrorists but Catholic doctors see a casualty on the battlefield, they don’t discriminate, a casualty is a casualty.