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Can. 1041 The following are irregular for receiving orders:
1/ a person who labors under some form of amentia or other psychic illness due to which, after experts have been consulted, he is judged unqualified to fulfill the ministry properly;
2/ a person who has committed the delict of apostasy, heresy, or schism;
3/ a person who has attempted marriage, even only civilly, while either impeded personally from entering marriage by a matrimonial bond, sacred orders, or a public perpetual vow of chastity, or with a woman bound by a valid marriage or restricted by the same type of vow;
4/ a person who has committed voluntary homicide or procured a completed abortion and all those who positively cooperated in either;
5/ a person who has mutilated himself [this includes vasectomy, castration, sterilization, etc.] or another gravely and maliciously or who has attempted suicide;
6/ a person who has placed an act of orders reserved to those in the order of episcopate or presbyterate while either lacking that order or prohibited from its exercise by some declared or imposed canonical penalty.
(cf. the commentary on this canon)
This Instruction Concerning the Criteria for the Discernment of Vocations with regard to Persons with Homosexual Tendencies in view of their Admission to the Seminary and to Holy Orders by CONGREGATION FOR CATHOLIC EDUCATION, is an example of how Bishops, major superiors and all relevant authorities are to carry out an attentive discernment concerning the suitability of candidates for holy orders, from the time of admission to the seminary until ordination. In one section it states
The spiritual director has the obligation to evaluate all the qualities of the candidate's personality and to make sure that he does not present disturbances of a sexual nature, which are incompatible with the priesthood. If a candidate practises h*m*sexuality or presents deep-seated h*m*sexual tendencies, his spiritual director as well as his confessor have the duty to dissuade him in conscience from proceeding towards ordination.