Before the sixteenth century most Catholic priests were trained by apprenticeship, if at all. A boy attached himself to a parish priest, learned the liturgy by repetition, and was ordained by a bishop who had often never examined him. The results were what the Reformation polemicists said they were, and the Council of Trent did not dispute the diagnosis.
Trent's answer was the seminary: a residential institution in every diocese, taking candidates young, teaching them scripture, theology and the languages, and keeping them under a regime of discipline and prayer until they were ordained or dismissed. The word means seed-bed.
The formative claim is more interesting than the academic one. A seminary is not principally a school; it is an extended assessment. Staff live alongside the candidates for years, and the purpose of that proximity is to allow judgements about character that an examination cannot make, and to allow rejection. The Second Vatican Council's decree on priestly training restated this and added psychological and pastoral dimensions.
The model spread far beyond Catholicism. Protestant divinity halls, Orthodox academies, and in structure though not in content the modernising yeshiva and the reformed madrasa all adopted the residential, staged, credentialing shape, largely in the nineteenth century and often in explicit imitation.
The characteristic failure is structural rather than incidental, and it is worth stating without euphemism. An enclosed institution that trains, assesses and promotes its own future supervisors, in which the people making judgements about a candidate's fitness are the same people responsible for his formation and are answerable to a superior who wants ordinations, is badly configured to notice what it would be costly to notice. Reviews of the Catholic abuse crisis in several countries have said so in those terms. Seminary numbers in Europe and North America have in any case collapsed to a fraction of their mid-twentieth-century level, and most of the institutions Trent's rule created have closed or merged.