69 This achievement in the erection and management of Seminaries for the education of future priests deserves all praise. But it would be of little avail, were there any lack of care in the selecting and approving of candidates. In this selection and approval, all who are in charge of the clergy should have some part: superiors, spiritual directors and confessors, each in the manner and within the limits proper to his office. They must indeed foster and strengthen vocations with sedulous care; but with no less zeal they must discourage unsuitable candidates, and in good time send them away from a path not meant for them. Such are all youths who show a lack of necessary fitness, and who are, therefore, unlikely to persevere in the priestly ministry both worthily and becomingly. In these matters hesitation and delay is a serious mistake and may do serious harm. It is far better to dismiss an unfit student in the early stages; but if, for any reason, such dismissal has been delayed, the mistake should be corrected as soon as it is known. There should be no human consideration or false mercy. Such false mercy would be a real cruelty, not only towards the Church, to whom would be given an unfitted or unworthy minister, but also towards the youth himself; for, thus embarked upon a false course, he would find himself exposed to the risk of becoming a stumbling block to himself and to others with peril of eternal ruin.
Source: Ad Catholici Sacerdotii (Vatican.va)