More proof, as if any were needed, that one lifetime is hardly enough to exhaust one's curiosity. After reading the story of the Afghan museum, and the preservation of the treasures of Kushan kings and Bactrian craftsmen, I googled around a bit, and found this essay:
Bactrian, the ancient language of Bactria in northern Afghanistan, is unique among the Iranian languages in being written by means of the Greek alphabet --- a legacy of the conquest of Bactria by Alexander the Great in the 4th cent. B.C. From this period onwards the Greek language, written in the Greek script, was for a long time the exclusive language of culture and administration in Bactria. When Bactria was overrun by nomadic peoples from the north, its new rulers, the Kushans, at first continued the use of the Greek language for administrative purposes, but soon they came to use the Greek script to write the local language, Bactrian. A crucial moment in the history of this language was the decision of the Kushan ruler Kanishka to adopt Bactrian as the language of his coinage. After the first issues of Kanishka, Greek disappears from the coinage once and for all, to be replaced by Bactrian.
During the first centuries of the Christian era, Bactrian could legitimately have been ranked amongst the world's most important languages. As the language of the Kushan kings, Bactrian must have been widely known throughout a great empire, in Afghanistan, Northern India and part of Central Asia. Even after the collapse of the Kushan empire, Bactrian continued in use for at least six centuries, as is shown by the ninth-century inscriptions from the Tochi valley in Pakistan and the remnants of Buddhist and Manichean manuscripts found as far away as the Turfan oasis in western China. The career of Bactrian as a language of culture thus lasted for close to a thousand years.
Buddhists, Manicheans, and later Zoroastrians make an appearance.