The booklet essay talks mostly about the libretto of Messiah, and how the librettist, Charles Jennens (who also wrote Handel's Saul and Belshazzar) had the idea to tell the story of Christ using selections of text from the Old and New Testaments. Jennens' aim was to make the case for fundamentalist Christianity, and I'm not using "fundamentalist" as a slur, but just in a descriptive way: Jennens wanted to literally get back to the fundamentals of religion, the basic ideas about who Jesus was and what his significance was. Using the Old Testament, in particular the prophecies of Isaiah, for a lot of the text was part of that, since it was meant to re-inforce the idea that the whole Bible was in some way or another leading up to the story of Jesus. Jennens saw Messiah as a way of counteracting all the Enlightenment free-thinking and doubting he saw going on around him. It was an interesting thing to do, the opposite of preaching to the converted: it would be like Mel Gibson making The Passion of the Christ for people who didn't necessarily believe in the literal truth of the story.
But Jennens wasn't entirely satisfied with what Handel did with Messiah. He wrote to a friend that Handel had written at least some music that was unworthy of the great subject:
He hath made a fine Entertainment of it, tho’ not near so good as he might and ought to have done. I have with great difficulty made him correct some of the grossest faults in the composition, but he retained his overture obstinately in which there are some passages far unworthy of Handel but much more unworthy of the Messiah.
Apart from the overture, I suspect Jennens might not have been happy with Handel's injection of humour, even satire, into parts of the work, most famously when he sets the words "All we like sheep have gone astray" by having the chorus imitate the bleating of sheep. And of course Handel famously incorporated obviously secular music into the piece, like "For Unto Us a Child is Born," which re-uses a theme from an Italian aria he'd written years earlier. More than that, Handel's Messiah really doesn't emphasize the doctrinal points that Jennens wanted to emphasize. The Nativity is dispensed with in about two minutes, while the longest aria in the work -- the emotional core of the whole evening -- is "He Was Despised," which isn't about the crucifixion or even necessarily about Jesus at all; it's about the scorn and mistreatment that a prophet must suffer in his own time (the words are from Isaiah). In what words he chose to set as arias and what words he dispensed with in recitative, and in the moments he chose for the really Big Moments, Handel came up with a work that is far less didactic and more dramatic than Jennens was probably expecting; the subject of Jennens' Messiah is Jesus, but the subject of Handel's Messiah is regular human beings and their relationship to Christianity and religion in general; that's why the big soprano aria, "I Know That My Redeemer Liveth," is set to lines about personal belief, not doctrine.
Handel was a religious man in his own way, but he was basically a theatre person and Messiah is basically a dramatic work in a way that most religious oratorios are not; that, I think, helps explain why it is the most popular religious work of classical music, because it gives us religion and belief on a very personal level. The Jacobs recording does a good job of conveying that, because Jacobs is a conductor who is willing to be brash and even vulgar if it'll make a strong dramatic point, and moments like the bleating sheep or the "fire" music in "But Who May Abide" are played for all their theatrical value.