The premise of Victor H. Mair's wide-ranging article is that written Chinese emerged not as transcribed speech, but rather as a special, radically shortened cipher with its own grammatical and expressive conventions. He calls this written form Literary Sinitic (LS) and finds the disparity between it and any form of spoken Chinese, which he refers to under the general heading of Vernacular Sinitic (VS), is of a wholly different nature than the contrast between written Latin and any modern written or spoken Romance language. Indeed, he argues, Literary Sinitic remained incapable of serving as a means of recording spoken Chinese or any other language. Thus, for Mair, the question becomes: How did vernacular written forms emerge in a milieu in which Literary Sinitic dominated intellectual life? He finds the earliest instances of written Vernacular Sinitic occur typically in Buddhist texts. He believes the Buddhist emphasis on teaching through the local dialect (desa bhasa) was a major impetus for the development of written vernacular, but concludes it is difficult to determine exactly which aspects of Buddhism had the greatest influence on the slow maturation of written Vernacular Sinitic.And here's Mair's conclusion.
We have seen how, under the probable influence of the Indian concept of desa bhasa brought to China by Buddhism, numerous peoples in East Asia created a whole series of written vernaculars. While Chinese authorities stubbornly resisted recognition of any of their own vernaculars as a national language--probably due to the extremely high prestige and power of LS--the Buddhists used the vernacular liberally in their own writings. Once proffered as a functional alternative written language, use of the vernacular steadily grew until, by the late Ming-early Ch'ing [= Qing], it is likely that as many books were being printed in vernacular or a heavily vernacularized literary style as in LS, not withstanding the censure and ridicule of strait-laced scholars. Finally, even the Manchus, who already had their own written national language, which was swiftly dying out because of pervasive sinicization, yielded to the idea that their Sinitic subjects, too, needed a national language keyed to one of the spoken vernaculars. After the agitation of the May Fourth Movement [in 1919] led by progressive Chinese intellectuals and students, many of whom were exposed to radical ideas about language and other aspects of culture and society through the window of Japan, kuo-yü [= guoyu, Mandarin] was publicly proclaimed the official written language of the nation. This marked the formal end of the multimillennial separation between book language (shu-mien-yü [= shumianyu]) and spoken language (k'ou-yü [= kouyu]) in China.SOURCE: Victor H. Mair, "Buddhism and the Rise of the Written Vernacular in East Asia: The Making of National Languages," Journal of Asian Studies 53 (1994): 705, 707-751
That Buddhism played a crucial role in the evolution of the written vernacular throughout East Asia is beyond any doubt. The question remains, however: Which aspect of Buddhism was responsible for these momentous changes? Was there some religious doctrine belonging to Buddhism that fostered the written vernacular? Was it the fondness for storytelling, preaching, and public speaking by the early Buddhists in the language of the people? Did the ostensible orality of Buddhist scripture have anything to do with the origins of the written vernacular in China? Was the fact that most of the early translators of Buddhist texts into Sinitic were foreigners with a poor command of the literary language a significant factor? And did the phonological sophistication of Indian linguistical science lend credibility to the spoken vis-à-vis the written? What of the elaborate, rigorously defined Indian traditions of chanting and recitation? And may the social values, institutions, and position of Buddhism have contributed to the rise of the written vernaculars? Last but not least, did Buddhist practice have anything to do with the validation of the vernacular? Perhaps I have entirely overlooked some vital facet of Buddhism that contributed to this process. In the end, Buddhist support for the written vernacular may best be identified as a complex combination of diverse factors, all of which were determined by an integrated socioreligous ideology.
UPDATE: The comment thread on this at Language Hat is most interesting.