How did a sixth-century rulebook for monks end up shaping medieval Europe?

Because it took a moderate path, and because an emperor helped it spread. Benedict of Nursia wrote his Rule around AD 530 for monks living communally under an abbot. Compared with other precepts, it takes a moderate path between individual zeal and formulaic institutionalism, and because of this middle ground it became widely popular. Charlemagne had it copied and distributed to encourage monks throughout western Europe to follow it as a standard. Beyond religion, it became one of the most important written works to shape medieval Europe, embodying the ideas of a written constitution and the rule of law.

Monks were not new. Christian monasticism first appeared in the Egyptian desert, where monks led by Saint Pachomius formed the first communities under an abbot, from the Aramaic abba, father. Benedict came later, in the West. Upset by the immorality of society in Rome, he gave up his studies at fourteen and lived as a hermit in a cave near Subiaco. Disciples gathered around him, and he founded Monte Cassino in 529, where he wrote the Rule near the end of his life.

What does the Rule ask? Its spirit is widely summed up as ora et labora, pray and work, though that motto is not stated in the Rule itself. Monks must do daily manual labour, never less than five hours a day. No monk may own anything without the abbot's leave, but the abbot must supply every necessity. The abbot is to call the brothers to council on all important affairs. Guests are to be received with all care and hospitality, for it is in them that Christ is received.

Benedict probably never planned an order. There is no evidence that he intended to found one in the modern sense: his Rule guided individual, autonomous communities, and all Benedictine houses still remain self-governing.