Good post. But since you identify it as Sproul's, it's no wonder...
Fortunately, I've installed a Greek keyboard on my laptop, so we can see:
ΙΗΣΟΥΣ ΧΡΙΣΤΟΣ
Ιησους Χριστος
Jesus Christ
Yes, R.C. here is referring to what are called "nomina sacra", which appear to be the first abbreviations, which were created by Christians, just as the "Book" (bound "codex", rather than scroll) was invented by Christans. However, it is an oversimplified description of the nomina sacra ("sacred names").
I suspect they were invented to save space on vellum, which wasn't all that prevalent (scribes would often scrape the text off a piece of vellum to reuse it, resulting in "palimpsests", which is an interesting topic in itself). Words related to God (God, Father, Heaven, Christ, Lord, Holy, Spirit, etc.) were used commonly in Scripture, and it would save space if all these words were reduced in length.
To make a nomem sacrum, the scribe would take the first (or first two) and last letters of the word, and put a horizontal bar over top to signify it as a nomen sacrum, and so we would get:
___
ΘC - θεος (God)
___
ΥC - Υιος (Son)
___
ΚC - Κυριος (Lord)
____
ΠΝΑ - Πνεύμα (Spirit)
I remember once being in a Roman Catholic Church, and in the front of the church was an image of a portion of Greek scripture, and you could see the nomina sacra in the text.
Yes, I was reminded of that myself, and was going to bring it up myself
ΙΧΘΥΣ - "ixthus" ("fish")
Ιησους - Jesus
Χριστος - Christ
Θεου - God's
Υιος - Son
Σωτηρ - Saviour