I wrote in my introduction to the Encyclical Lumen Fidei (published by Mustard Seed Bookshop) that one should avoid the temptation to do “source analysis” on the Encyclical, trying to guess which bits were in Benedict’s draft and which bits Francis added.
But sometimes the source is so obvious that you cannot avoid the conclusion: “This was in the original draft by Benedict”. I am re-reading “Introduction” in preparation for teaching my Anima Education course on the Creed, and it is fascinating to see all the points where it connects with Lumen Fidei.
Here is a clear case. Chapter Two of Lumen Fidei is entitled “Unless you believe, you will not understand (cf. Is 7:9).” There then follows a rather interesting linguistic analysis of Isaiah 7:9 “If you will not believe, you will not be established” focusing on the Septuagint translation “If you will not believe, you will not understand”. He then goes on to construct a strong argument for the relationship of faith to truth.
It isn’t “word-for-word”, but it is “idea-for-idea” in line with a passage in Joseph Ratzinger’s 1968 classic “Introduction to Christianity” in a section entitled “Faith as standing firm and understanding” (p69 following in the Ignatius Press English edition). I won’t go into detail here, just check it out for yourself if you have time. It is fascinating to see how, after 45 years, his ideas are still fresh, relevant and inspiring.
You could note too that he did the same thing with the Encyclical Spe Salvi, by incorporating into it his ideas about Purgatory from his 1976 book “Eschatology“. Together, I believe that “Introduction” and “Eschatology” are the two most important theological works of his pre-papal career.