Linguists have their own technical way of distinguishing “dialects” from “languages” proper. It has something to do with numbers of speakers and levels of shared intelligibility. But ask more historically minded linguists what the difference really amounts to, and they’ll point to two things a language has that a dialect lacks: an army and a navy. To this list Nicholas Ostler, author of the decorous “Ad Infinitum: A Biography of Latin” (Walker & Company, 400 pages, $27.95) might add a third: a church. Better yet, a church that has an army and a navy, and isn’t afraid to use them.

The story of Latin’s success is, of course, more complicated than this smart little triad suggests. The third member of the list, the church wasn’t necessary to secure Latin’s place in the world until well into the language’s adolescence. And to be perfectly frank, a navy wasn’t all that critical for Latin either. The Roman army pretty much took care of the military business on its own. Under its brutally efficient authority, Latin went from being, in the fourth century before the common era, only one of several languages spoken on the Italian peninsula to being, in the fourth century of the common era, the one language spoken throughout a region bounded by Britain in the West and Moldova in the East. Certain parts of the Roman Empire never took to Latin, but only because those parts — North Africa, the Levant, present-day Turkey — had a language that even the Romans (well-educated ones, at least) recognized as nobler than their own: Greek.

When the Roman Empire collapsed at the hands of successive waves of German invaders, one might have suspected the Latin empire to collapse with it. But two things intervened. First, the Germanic invaders of the Roman empire were so outnumbered that even where they succeeded, their languages never took root. (Mr. Ostler estimates that “in the fifth and sixth centuries at most half a million German speakers came to settle among 20 million speakers of Latin.”) Second, the German invaders themselves proved remarkably accommodating to the existing culture, and even to Latin. “The glory of the Goth is the maintenance of civilization,” said Cassiodorus, the minister of the Gothic king Theoderic. Turns out that he meant it. If only Attila the Hun had been as polite.

But a congenial class of ruling Germans was not, Mr. Ostler writes, the whole story. The church had been like a fortress guarding the Latin language since the Roman emperor Constantine established Christianity as the religion of the empire in mid-fourth century C.E. The invading Germans were potential recruits, and when the church successfully converted a goodly number of them, it baptized them into God and Latin, in one fell swoop. For the church was surely not going to allow the Mass to be celebrated in Ostrogoth.

In the later years of the first millennium, as local vernaculars — which would become our French, Romanian, Spanish, inter alias — began to develop all over the erstwhile empire, the church insisted upon Latin, extending its life in the process. In 880, Pope John VIII wrote to Svatopluk, King of the Moravians and owner of a deliciously un-Latin name, to tell him that “In all the churches of [his] land the Gospel must be read in Latin because of its greater dignity.” Only afterward, conceded the pope with some dismay, “should it be announced in the Slav language to those who do not understand Latin words, as seems to be done in some churches.”

The church’s defense of Latin put it in a curious position. For Latin, of course, was the language of the quintessential — a word whose curious Latin etymology Mr. Ostler nicely traces to complicated metaphysical debates in the Middle Ages — pagan culture. Jerome, who was primarily responsible for translating the Bible into Latin (the so-called “Vulgate” edition) famously dreamt that at the gates of heaven he was scolded for having been more deeply devoted to Cicero than Christ. Augustine worried about his own loyalties too, but found a timely solution in a verse from Exodus: “And the Lord gave the people favor in the sight of the Egyptians, so that they lent unto them such things as they required. And they looted the Egyptians.” The Egyptians, on Augustine’s reading, were Latin-speaking pagans. Christians could take from them whatever was needed.

If Augustine and Jerome were troubled by their affection for Latin, Mr. Ostler needn’t himself worry. This is a fine book, but Mr. Ostler doesn’t have much fondness for Latin itself. Of a beautiful Ciceronian sentence, its syntax expertly calibrated, Mr. Ostler grumbles that the structure “is not really an aid to clarity of thought. The whole thing is contrived, and its meaning disguised rather than revealed by the form … [of] the sentence.” I could not disagree more. Has Mr. Ostler read a newspaper or — I’m sorry to say this, but since Mr. Ostler has cast this particular die I can’t resist — some of his own sentences? If only anyone still wrote like Cicero!

This is not to say we need requirements for higher education such as this one, established by Harvard in 1642: “When any [student] is able to read [Cicero] or such like classical Latin author… then may he be admitted into the College, nor shall any claim admission before such qualifications.” It is to say, however, that it’s a fine idea to remember just how deep was the mark that Latin left on the West. We tend to think of Latin as something that might season our dreary vernacular, per the cutesy suggestion of books such as “Put a Little Latin in Your Life.” But our very best prose and public oratory does not have just a “little Latin.” It’s Latin through and through — in tempo, structure, and diction. This goes for even our most ostensibly demotic fiction. When Philip Roth or Saul Bellow unspools a sentence that you think cannot go on one word longer, but somehow does, before it stops, turns back on itself, and leaves us in awe, surely neither has Cicero in mind. But as we catch our breath, we might spare a thought for the language that gave our own this capacity to ravish. I’m not sure Ostrogoth could have done the same.

Mr. Boyle teaches classics at the University of North Carolina at Chapel Hill. He last wrote for these pages on Herodotus.