Of course it's just a bit of trivia, a bit of odd background, that Peretti, who "kicked open the doors" for Christian fiction authors, was originally asked to write Left Behind, the most overwhelmingly successful example of the genre*.
Journalists and scholars generally just bring this up to talk about the broader context of Christian fiction, putting the LaHaye-Jenkins juggernaut into some kind of relationship to the sub-culture market that preceded it, and to talk about the working relationship between LaHaye, who had the idea and did the outlines of major events, and Jenkins, who did the work of coming up with characters, writing the stories, etc.
It's like Peretti is the Pete Best of this situation, and you have to think of this success anew in terms of contingencies.
But, what's interesting too, and probably worth thinking about, is how that turned-down offer highlights some aspects of the series and what made it successful, and shows how they weren't accidental but part of the plan all the time. Both the writes and academics sometimes talk about Left Behind like it's a surprise and like it succeeded how it did completely of it's own accord or because of some unforeseeable chemistry.
But, this little bit of back story and not-important trivia points to several important things which were part of the success that weren't accidents, but part of the plan all along.
There were several commitment, here, in the pre-history, that are important:
Peretti dominated in Christian sales. His first adult novel was on the Christian best-seller list for almost three years straight. There really wasn't anything else that did that. It's quite possible LaHaye thought "Peretti" simply because in the 80s and 90s, when you thought Christian fiction, you thought Peretti.
The fact Peretti was pitched the series suggests, it seems, that LaHaye wasn't at that early stage, interested simply in translating the apocalyptic theology into fiction.
He wanted a blockbuster.
2) Cross over:
Not only did Peretti dominate in the 80s and 90s, he was the only Christian author who had or appeared to have a real chance at crossing over into the secular market. He was compared to Stephen King and John Grisham, but not just as a knock-off, as per usual with Christian market writers, but as having the potential to appeal to those audiences.
He did, eventually, get several of his books made into movies and break onto the New York Times bestseller list (though that was after Left Behind and, arguably, had to do with a change in the market unrelated to Peretti, though the sentiment prior to that was that he had the potential to change things).
There were a lot of Christian writers in the 90s. The fact that LaHaye asked Peretti to help with the writing of these novels suggests that it's possible that LaHaye was aiming at crossover success all along.
3) Theological flexibility:
Peretti was criticized from a very early date for his theology. His ideas about prayer, and how it works, and demons, and how they're involved in everything, and "spiritual warfare" that isn't metaphorical, were not widely accepted among evangelicals. Pentecostals and Charismatic’s make a lot of evangelicals uncomfortable -- there's even a language to it, an everyday language that's used that's unique to those of Peretti's theological bent, which wouldn't be natural and wouldn't be accepted by Baptists like LaHaye.
Apparently that didn't bother LaHaye.
He apparently set certain a theological frame work -- e.g., which end times event would happen in which book and when, and that there's be (at least) one evangelical conversion experience staged in each book -- and was quite flexible about the rest of it.
Normally Left Behind is critique by non-Christians as being nothing but a theology-carrying device, and by Christians as being bad theology. The fact that LaHaye asked Peretti to write the books suggests that, for the most part, theology in any detail was not the point.
It's not incidental that Left Behind was a kind of Christian horror/thriller. It was conceived that way.
5) Politically right wing:
Peretti had a demonstrated commitment to narritivizing the culture war as a spiritual conflict, "literally demonizing," as Daniel Radosh points out, the ACLU, the New Age movement, pro-choice movement, etc.
Much more so than Jenkins.
Some of the politics of the Left Behind novels weren't inherantly necessary to a story about the pretribulation dispensationalist "secret" rapture, but LaHaye's attempt to draw Peretti in shows that they were part of what he wanted to do with the books (and may have been substantially more central than, say, details of the efficacy of prayer or the dispensationalist's understanding of soteriology).
In the final novels, the politics can sometimes seem incidental and are often idiosyncratic -- the first book's single passage on abortion, which is most just a thought that one of the characters has and doesn't connect to anything else, for example, and the fact that, in passing, it's mentioned that non-Christian pregnant women lost their unborn babies as fetuses were raptured -- but the fact that Peretti's politics, if not his actual Christianity, lines up with LaHaye's so closely, suggests that the cultural wars parts were at the heart of the books in their conception.
There may well be other committments suggested by the Peretti offer, but it seems to serve, at least, to show that certain apparently incidental facts about the books were not a product of how it worked out, were not just accidents, but were central to the idea of the books at their very inception.
It's possibly even in a league with Christian novels that were marketed to a general audience, before "Christian" became one of the many, many niche markets of the book industry. E.g., Ben-Hur, Pilgrim's Progress. It's difficult to find acutal numbers for those books -- we know, for example, that a 1912 printing of Ben-Hur was the largest single print edition in American history at 1 million, but beyond that what you read is "It outsold every book except the Bible until Gone With the Wind came out in 1936, and returned to the top of the list again in the 1960s."
But -- in 1912, there was one copy of Ben-Hur for every 95.3 Americans, and in 2001 there was one copy of Desecration, the 9th book in the LaHaye-Jenkins' series, for every 95.2 American.