Which language, revived from liturgical use, is the only one to have become a national tongue again?

Hebrew. It had not been anyone's everyday first language for something like seventeen centuries - it survived as the language of scripture, prayer and scholarship, read and written continuously but not used to argue about dinner - and it is now the mother tongue of millions.

Eliezer Ben-Yehuda is the name attached to it, and he was fanatical in a way that is usually softened: he refused to speak anything else to his son, who was raised in near-isolation from other children so that no other language could reach him, and reportedly did not speak until he was four. He also had to invent thousands of words for things the Bible never mentioned, building them from existing roots - ice cream, newspaper, bicycle, tomato.

The one-man version is a myth, though. The revival needed a generation of teachers who taught in Hebrew in Palestine's new schools, immigrants from many language backgrounds who needed a common tongue anyway, and eventually a state that made it official. Every other revival attempt - Cornish, Manx, Irish - has produced enthusiasts and school subjects rather than a country of native speakers, which is why this remains the only case.