Two Corinthians versus Second Corinthians: For most of the world, it's the former that's correct.
American politician, Donald Trump got up the ire of the American media, for referring to II Corinthians (written 2 Corinthians by the American standard) as 'Two Corinthians' /tuː kəˈrɪnθɪənz/. According to American Media, the correct pronunciation is 'Second Corinthians' /ˈsekənd kəˈrɪnθɪənz/. Thing is: most people in the English Speaking World say 'Two Corinthians'. At every mainstream non-American-based church I have ever attended, it has been referred to as Two Corinthians. You talk of 'Two Corinthians, Thirteen' for instance. Referring to 'Second Corinthians Thirteen' would likely have people asking whether or not you are referring to a second draft or a second serving.
In fact 'Two Corinthians' /tuː kəˈrɪnθɪənz/ is the correct pronunciation in British or received English. This is why most nations follow it. Perhaps the presence of a latin numeral 2 is behind the mass adoption of this, just as the Arabic numeral 2 might be behind the American convention.
Both conventions are a shortening, a summary of a book title. A stray example of such a practice can be seen in: Alice's Adventures in Wonderland, which is commonly shortened to Alice in Wonderland. We don't of course shorten it to Alice's Wonderland, although that might have emerged as a valid shortening.
In the case of Two Corinthians, the proper name of the letter is: ‘St Paul's Second Epistle to the Corinthians’ (based on Oxford) /seɪnt ˈpɔːlz ˈsekənd ɪˈpɪsl̩ tuː ðə kəˈrɪnθɪənz/. However, Americans might call it ‘The Apostle Paul's Second Letter to the Corinthians’, or just ‘Paul's Second Letter to the Corinthians’.
While we are at it, what about the Second World War? Many say it as: World War II /wɜːldwɔːˈtuː/. That itself hides the word: 'number': World War Number Two, but is perfectly correct as a reference term. In fact, it might be said using the word ‘number’ is entirely superfluous, in that case.