Lawmeli backstory reference post
Posting this separately cause dammit, I researched/wrote like a thousand words on this ridiculous character's background, I want to make sure it's accessible to other people who want to use some of it in their fics without having to read a whole back-and-forth meta conversation first!
This is all FROM THE BOOK, so technically applies only to Jacopo Lomeli and not Thomas Lawrence. That said, nothing that they say in the movie DISPROVES any of it, and given how closely the movie otherwise adheres to the book, I tend to assume that anywhere they don't explicitly change something, the assumption is it should be the same. THAT SAID, I do want to call out that movie heavily implies that the late Holy Father reigned for significantly longer than he did in the book, because in the movie, Benítez offered his resignation "in his late thirties" (to, presumably, the same Pope who just died), while in the book, he only offered to resign in January of the same year that the Pope died. While Benítez's age is never clearly established in the movie, him being younger than mid-50s would be remarkable and worthy of notice, which means that the late Holy Father had to have reigned for at least 15 years, and more probably 20+ (compare this to the book, in which he was elected after Ratzinger's retirement, and so presumably in 2013, and so likely died after only ~6-10 years in office, the book being placed in ~2021, give or take a couple years).
It's also worth noting that I remember someone pointing out at some point (but can't find it now, of course) that Thomas and Aldo have matching rings of office in the movie, which imply they were appointed by the same Pope (which almost certainly wouldn't have been the case in the book.). Anyway, those changes in general would have a lot more impact on Bellini's career than Lomeli/Lawrence's, but could still have had an effect on the landscape, so all the information below is probably but not canon for Lawrence.
Finally, before we dive in, I want to call out that we are never given any information at all on Lomeli's backstory before he joined the Church, which is an interesting and clearly deliberate character choice. We have significantly more early and familial backstory on Tedesco and Tremblay, who are relatively minor characters, than on our protagonist! We don't know what his parents did, if he had siblings, or even what region of Italy he comes from. The only history we have on him is his professional career—Lomeli has no identity outside the Church.
So, in the book, Lomeli's career went:
Professor of Canon Law at the Gregorian (70s) — Priest
(That he was a professor of canon law is certain—we don't know what university but it's likely it would have been the Gregorian. Era is not explicitly stated, but the book is placed roughly in ~2021 (give or take a year or two either way), and Lomeli is 75, which means he was born in the mid-40s. Which means he would have made professor at some point in the 70s.
Various diplomatic posts (80s) — Bishop
This is the part that we have the least evidence on, but we know that that Lomeli never served in a "ordinary parish" and it's incredibly unlikely that he would have achieved his next position without some lower level diplomatic experience. He also would have been promoted to be a titular archbishop for his next position (it's traditional, since 1978), so it's likely he was made a bishop before that.
Permanent Observer to the UN (90s) — Archbishop of Aquileia
The book says that he was "Apostolic Nuncio in the United States." However, there are two other references to him having served in New York, which means that he couldn't have been the Nuncio to the US, who serves in Washington DC. The only other possibility is that he was the Permanent Observer to the United Nations, who is also an Apostolic Nuncio, and is traditionally made a titular Archbishop. I went through all the Permanent Observers through history, and none of them held the rank of cardinal until after leaving the UN, so it's also almost certain he wasn't named cardinal until after this. The book specifically says he held this post in the 90s.
Secretary for Relations with States (00s) — Cardinal-Priest
One of things I think Harris does most amazingly in the book is fit in these little bits of world- and character-building so smoothly you barely notice them. In this case, the part I skated over was:
He picked up his pectoral cross from the nightstand and kissed it. John Paul II had presented him in person with the cross to mark his recall from New York to Rome to serve as Secretary for Relations with Foreign States. The Pope’s Parkinsonism had been terribly advanced by then; his hands had shaken so much as he tried to hand it over, it had dropped on the floor.
So, from this paragraph we know that he was promoted directly from his post to the UN to this higher post in the Secretariat of State. It's a job that (scrolling through the various priests who have held it) is usually accompanied by a rise to the cardinalate, and we know that Lomeli had to have been named a cardinal by John Paul II, because he remembers Ratzinger's "shy smile" at the moment his vote tally passed two-thirds, which means Lomeli must have been in the room during the voting. So it probably happened here (also consistent with him still wearing the same pectoral cross). We also know this had to have happened between 2001 (John Paul II diagnosed with Parkinson's) and 2005 (John Paul II's death). This position is often called the Vatican's "Foreign Minister" and there are only two positions higher than it in the Secretariat of State, those being the Secretary of State himself, and the Substitute for General Affairs.
Secretary of State (10s) — Cardinal-Bishop
And so, now here's the big question. Was Lomeli made Secretary of State under Ratzinger, or under the Late Holy Father? For the life of me, I cannot tell from the book. The only real info we get is:
When the traditionalists had taken control of the Congregation for the Doctrine of the Faith in the nineties, he had kept his head down and got on with his work as Papal Nuncio in the United States. Twenty years later, when the late Holy Father had decided to clear out the old guard and had asked him to step down as Secretary of State, he had nevertheless served him loyally in the lesser role of dean.
So, we know that Lomeli was counted as part of the "old guard"—but given that it's clear that he'd already been high in the Curia under John Paul II, this makes sense regardless of whether he served as Ratzinger's Secretary of State, as well. And we know that the Late Holy Father asked him to step down at some point in the 2010s—that being twenty years later than the 90s—but if we're assuming that the Nameless Pope in the book was elected the same year as Francis (e.g., 2013), and that the Conclave is set sometime post 2019 (the only hard "modern" date we have in the book is that Vincent was named Archbishop of Kabul in 2018, which is implied to have been fairly recent), then this... also doesn't give us any new information.
However, we are also told that Lomeli was only Secretary of State "briefly," which means that he couldn't have possibly been Secretary of State for Ratzinger's entire 8-year pontificate, or even probably for most of it. (Since he was diagnosed with cancer "a few years ago," him being Secretary of State only briefly also implies that him being asked to resign probably wasn't related to his cancer—I say this only because I've seen that headcanon a lot in fanfiction.
So I can see two scenarios that both make an equal amount of sense with the information we're giving:
Ratzinger's previous Secretary of State stepped down or died in the last year of his pontificate, when his own health was already very poor, and he appointed Lomeli as a reliable servant who wouldn't make any waves while "managing" (heh) his retirement and the transition from his pontificate to the next. Lomeli then hung on for several months into the next Pope's administration before being asked to resign (this is actually very common—there hasn't actually been a single instance in the last sixty years where the Vatican Secretary of State wasn't at least temporarily reappointed by the next Pope, even if it was only for a few months).
The Late Holy Father decided to get rid of Ratzinger's Secretary of State as quickly as possible, but didn't want to completely overhaul the entire dicastery, and so appointed Lomeli, as the highest-ranking liberal-leaning centrist in the Curia at that time, as his Secretary of State to provide some institutional continuity. He then asked him to step down a year or two later and put Bellini in his place, hoping that would enable him to make greater progressive strides.
Personally, I lean toward the latter, but it could really go either way. It does absolutely add a really interesting level of complexity to Aldo and Thomas's friendship, though—especially given Aldo's whole "I never knew you were so ambitious" line!