I am writing an extensive commentary on the book of Matthew. On the topic of plagiarism, many of the comments I would like to use are listed in several sources verbatim with no citation on any of them. How does one go about citing in this situation?
If you're quoting scriptures, the only citation you need to make is which version you're using. That is usually done on the copyright page and sometimes mentioned in the introduction or foreword.
If you're quoting someone else's commentary, do your very best to find the original source. Sometimes you can Google the quote and it'll pop up. Unfortunately, some quotes are attributed to a variety of people and it's difficult to find the original.
If you can't find a source, you cite it as "Source Unknown" within the text. Then on the copyright page, make a note that explains you've done your best to track and cite sources correctly, but if a reader knows the source of a quote listed as unknown, to please contact you. Then double-check their source information to make sure it's correct and add the new source in the next printing.
It also helps to keep a log of all the different ways/times you've tried to find the source and the results. That way, if someone sues you later, you can prove you did due diligence.