When Pius V became pope in 1566, the Council of Trent had closed only three years earlier, leaving its sweeping program of reform still largely unenacted across a Latin Church that had spent decades responding piecemeal to the Protestant Reformation. Pius made implementation his central project. In 1566 he issued the Roman Catechism, a summary of doctrine intended for parish priests rather than scholars, meant to standardize what ordinary Catholics were actually taught at a moment when confessional lines across Europe were hardening. He followed it with a reformed Roman Breviary in 1568, consolidating the daily cycle of psalms, readings and prayers that clergy were bound to recite, and then, in 1570, with the Missal that took his name, an attempt to fix the text and rubrics of the Mass itself into a single authorized form for the whole Latin Church, superseding the patchwork of local uses that had multiplied across the medieval centuries.
The Missal's bull of promulgation, Quo Primum, presented the new text not as one option among several but as binding, with only rites that could show at least two centuries of continuous local use permitted to continue alongside it. That form endured, with revisions, until the Second Vatican Council's Missal of Paul VI replaced it four centuries later in 1970, and it survives today as the Tridentine Mass, the older of the Roman Rite's two authorized forms. Pius's own background as a rigorous Dominican theologian and former head of the Roman Inquisition shaped the reforms' character throughout, favoring uniformity and doctrinal precision over the regional variation the medieval Church had tolerated.