So what's this "Congrefs" thing all about?

Well, it's not an italic 'f' at all: It's a "long s", also called a "medial s". If you look closely at a facsimile of an actual document which uses it -- it faded out in the nineteenth century -- you'll notice that there's no crossbar as in an "f", or at most there's just a little nub there.

The "long s" came about during the Middle Ages, along with "&", ligatures, and a lot of other typographical and calligraphical oddities. Sometimes it's easier to write it than the round "s" familiar to us today. When you're copying the entire Bible by hand, convenience matters. Just as importantly, it looks nice. Aesthetics have always been a big thing with people who take typography and calligraphy seriously.

It's called the "medial s" because the general rule (never religiously applied) was that it was only used in the middle or beginning of a word; hence the "round s" at the end of "Congrefs". The "round s" is sometimes called the "Terminal S" for that reason.

The "long s" found its way into typography the same way ligatures did: That's how writing was done, so they did it that way. Furthermore, it looked cool.