Well, he's a historian. Historians spend a lot of time reading original sources. A lot of that reading is pretty dull, I imagine. The internet makes research much easier these days, but there are always new things coming to light that aren't on the internet. It takes a lifetime to acquire all the knowledge these guys have, but for them it's just the job they do. They come to know the facts of history the way masons know mortar and bricks. Intimately.
Ostensibly, all of us who teach the faith had to learn the accumulated knowledge of many centuries in order to be a reliable teacher -- a bard, a griot, a shanahy, the one who chants our tribal lays, who initiates the young and the outsider into the People of God, who defines the boundaries of teaching and practice. Ah, the lyf so short, the craft so long to lerne. When you have finished all the secondary education, university education, and graduate education we require, most people still have only a sketchy mastery of what-all there is to know.
And some stop there. They spend the next few decades regurgitating the slogans and crib notes they remember while concentrating on motivational stuff or administration or conducting worship. Which is sad. But some slog on, continuing to read the old teachers (or at least, about the old teachers), continuing to learn, continuing to flesh out one's understanding of the whole Christian story.
I find myself able to talk easily across that whole range now. I know so much more than I did when I graduated with my MDiv, although I'm still using what I learned in that process. Plus, I have forty-ump years of life experience that I can cite to illustrate what I know. And I find myself still wanting to teach, because once you know something, you want to share it with others. I want to help others acquire (hopefully with less labor) some part of all that I have managed to heap up and sort out.
Why? Don't all such pursuits end in dust and silence? Much study is a weariness of the flesh, saith the Preacher. As for knowledge, it shall pass away, said the Apostle. Yes, it will. I may, before my death, lose my grasp on all that I now know -- so it's important to utter it while I still can. And after my death, I won't need to master what the old teachers said, because I'll be able to ask directly, and what I need to know will come without the labor of study. And again, if we are to "redeem the time," then we need to pass on what we know -- to make the way easier for the disciple of today and to raise up good teachers for tomorrow.
I don't teach, or write, in order to show off what I know. I only know that I don't really know what I know until I have communicated it to someone else. In teaching, I fix my learning, and retain it. My delight is in finding someone else who really just wants to know things; not so I can be the teacher and he or she the pupil, but so we can revel in the sheer joy of how our knowing something has enlarged our universe, and knowing something together means that we are not alone.