That he who is to be ordained a Bishop must be steadfastly resolved to observe the canons, otherwise he shall not be ordained.
When we recite the psalter, we promise God: “I will meditate upon thy statutes, and will not forget thy words.” It is a salutary thing for all Christians to observe this, but it is especially incumbent upon those who have received the sacerdotal dignity. Therefore we decree, that every one who is raised to the rank of the episcopate shall know the psalter by heart, so that from it he may admonish and instruct all the clergy who are subject to him. And diligent examination shall be made by the metropolitan whether he be zealously inclined to read diligently, and not merely now and then, the sacred canons, the holy Gospel, and the book of the divine Apostle, and all other divine Scripture; and whether he lives according to Gods commandments, and also teaches the same to his people. For the special treasure (οὐσία) of our high priesthood is the oracles which have been divinely delivered to us, that is the true science of the Divine Scriptures, as says Dionysius the Great. And if his mind be not set, and even glad, so to do and teach, let him not be ordained. For says God by the prophet, “Thou hast rejected knowledge, I will also reject thee, that thou shalt be no priest to me.”
Ancient Epitome of Canon II.
Whoever is to be a bishop must know the Psalter by heart: he must thoroughly understand what he reads, and not merely superficially, but with diligent care, that is to say the Sacred Canons, the Holy Gospel, the book of the Apostle, and the whole of the Divine Scripture. And should he not have such knowledge, he is not to be ordained.
Whoso is to be elevated to the grade of the episcopate should know…the book of the Apostle Paul, and the whole divine scripture and search out its meaning and understand the things that are written. For the very foundation and essence of the high priesthood is the true knowledge of holy Scripture, according to Dionysius the Great. And if he has this knowledge let him be ordained, but if not, not. For God hath said by the prophet: “Thou hast put away from thee knowledge, therefore I have also put thee away from me, that thou mayest not be my priest.”
The persecution of the Iconoclasts had driven all the best Christians into hiding, or into far distant exile; this had made them rustic, and had taken from them their taste for study. The council therefore is forced to be content with a knowledge of only what is absolutely necessary, provided it was united with a willingness to learn. The examination with which the ceremony of the ordination of p. 557 bishops begins seems to be a remains of this discipline.
The Synod teaches in this canon that “all Christians” will find it most profitable to meditate upon Gods justifyings and to keep his words in remembrance, and especially is this the case with bishops.
And it should be noted that formerly not only the clergy, but also the lay people, learned the Psalms, that is the whole Psalter, by heart, and made a most sweet sound by chanting them while about their work.
But as time went on, little by little this pious custom of reciting the Psalter and of imposing its recitation and a meditation thereon at certain intervals, slipped away to the clergy only and to monks and nuns, as to those specially consecrated to the service of God and to meditation upon the divine words, as Lupus points out. And from this discipline and practice the appointment of the Ecclesiastical or Canonical Office had its rise, which imposes the necessity of reciting the Psalms at certain intervals of time.
This canon is found in the Corpus Juris Canonici, Gratians Decretum, Pars I., Dist. xxxviij., C. vj., in Anastasiuss translation.