Several books ago Merlin mused that if some intelligence were watching over Safehold, it would have intervened centuries ago when the church started going off the rails, and especially when gunpowder was invented. So it's clear that for the last few centuries there's been no active supervision beyond the mortals of the Inquisition and the strictures of the Writ and the CoGA to keep Safehold within the static technological matrix that Langhorne envisioned.
The millennial visit from the archangels will change that - either for better or worse, depending on whether you're a TL or a techie. (Let's put the Reformists in the middle and assume they could jump either way.)
If the AI or archangel in stasis or whatever method the returning "archangels" use denounces the progress of the past century (since gunpowder) as violations of the Writ and tries to slam the genie back in the bottle, there's going to be a rather noisy and violent result, with mushroom clouds as the Rakurai obliterates manufactories across Safehold, destroys Tellesberg, Siddar City, and quite possibly most of the locations that produce "high tech" devices across the planet. Keep in mind that the "rot" has spread, just as Merlin intended!
When the dust clears, Safehold will literally be worse off than on the Day of Creation tech-wise, with quite a lot of its infrastructure damaged as well. You can pretty much write off humanity as a space-going species for several more centuries, (if then) and only after he Rakurai is destroyed and people become brave enough to once again experiment with things that the myths say caused God and the archangels to wreak destruction across the planet. That's unlikely to happen if Merlin, Nimue, or some of the inner circle don't survive or if Nimue's cave is discovered.
Now if the return of the archangels was always intended to declare that mankind has waited long enough, the Gbaba haven't found them and so they likely won't until humans go hunting for
them, and so the
archangels proclaim the Big Reveal, things will suddenly be a
lot easier for our heroes. Plus the archangels will send Clyntahn off his eternal punishment, (much to Merlin's chagrin) and the life span of the existing inquisitors will be measure in minutes!
Now which of these possible outcomes would make it easier to convince Safehold of the Big Lie and prepare humanity to wipe out the Gbaba? Hmm...
Of course neither of these outcomes would be very satisfying, so I'm sure RFC has something
much more clever up his sleeve. (Plus I'm almost
always wrong...)