These changes, traced by historical linguistics, are not unusual in the development of languages but appear to have taken place unusually quickly in Irish. According to one theory given by John T. Koch, these changes coincide with the conversion to Christianity and the introduction of Latin learning. All languages have various registers or levels of formality, the most formal of which, usually that of learning and religion, changes slowly while the most informal registers change much more quickly, but in most cases are prevented from developing into mutually unintelligible dialects by the existence of the more formal register. Koch argues that in pre-Christian Ireland the most formal register of the language would have been that used by the learned and religious class, the druids, for their ceremonies and teaching. After the conversion to Christianity the druids lost their influence, and formal Primitive Irish was replaced by the then Upper Class Irish of the nobility and Latin, the language of the new learned class, the Christian monks. The vernacular forms of Irish, i.e. the ordinary Irish spoken by the upper classes (formerly 'hidden' by the conservative influence of the formal register) came to the surface, giving the impression of having changed rapidly; a new written standard, Old Irish, established itself.