73 These are severe words, Venerable Brethren, yet still more dreadful is the responsibility which they declare, a responsibility which justified the great Bishop of Milan, St. Charles Borromeo in saying: "In this matter, my slightest neglect can involve me in very great sin." Listen to the warning of Chrysostom whom We have just quoted: "Impose not hands after the first trial nor after the second, nor yet the third; but only after frequent and careful observation and searching examination"; a warning which applies in an especial way to the question of the uprightness of life in candidates to the priesthood: "It is not enough," says the holy Bishop and Doctor St. Alphonsus de Liguori, "that the Bishop know nothing evil of the ordinand, but he must have positive evidence of his uprightness." Hence, do not fear to seem harsh if, in virtue of your rights and fulfilling your duty, you require such positive proofs of worthiness before ordination; or if you defer an ordination in case of doubt; because, as St. Gregory the Great eloquently teaches: place the weight of the building upon them at once. Delay many days, until they are dried and made fit for the purpose; because if this precaution be omitted, very soon they will break under the weight"; or, to use the short but clear expression of the Angelic Doctor: "Holiness must come before holy orders . . . hence the burden of orders should be placed only on walls seasoned with sanctity, freed of the damp of sins."
Source: Ad Catholici Sacerdotii (Vatican.va)