6th Ecumenical Council, Canon 64
That a layman must not publicly make a speech or teach, thus investing himself with the dignity of a teacher, but, instead, must submit to the ordinance handed down by the Lord, and to open his ear wide to them who have received the grace of teaching ability, and to be taught by them the divine facts thoroughly. For in the one Church God created different members, according to the utterance of the Apostle, in interpreting which St. Gregory the Theologian clearly presents the right procedure in these matters by saying: "Let us have respect for this procedure, brethren, and let us observe it.
First, let one man be a listener, as the hearing recipient; another, the tongue; another, a hand; another, something else; let one man teach, and let another man learn; and after short periods, as touching one who learns in a state of obedience, and one who leads the chorus in hilarity, and one who renders service in cheerfulness and willingness, let us not all be a tongue, heeding the most apt saying: "Let us not all be Apostles; let us not all be Prophets; let us not all be Interpreters" (1 Cor. 12:29), and after somewhat: "Why are you making out that you are a shepherd, when you are a sheep? Why are you becoming a head, when you happen to be a foot? Why are you attempting to be a general, when you are placed in the ranks of (ordinary) soldiers? And from another quarter Wisdom bids: "Be not hasty in words; vie not with a rich man when thou art indigent" (Prov. 23:4); nor seek to be wiser than the wise. If anyone be caught disobeying the present Canon, let him be excommunicated for forty days.Interpretation
The present Canon prohibits any layman from teaching openly and in church as a teacher; instead he should rather himself be taught by those who have received the gracious gift of teaching. For, just as there are various members belonging to one and the same body, as St. Paul says, so and in like manner there are various persons in the one Church, in the order in which placed each of them. Hence in interpreting this saying of the Apostle’s (in his Homily concerning due order in discussions) he says that one person in the Church must be an ear, another a tongue, another a hand, and another some other member; and neither must all of them be a tongue, or, in other words, teachers, nor must all of them be Apostles, nor all of them Prophets. So, O man, being a sheep, why are you trying to make yourself out to be a shepherd? Being a foot, why are you trying to be a head? Being a soldier, why are you undertaking to be a general? or a leader of soldiers?
Solomon, too, says: "Be not glib of speech and ready to say things; nor, when poor, quarrel with the rich; nor seek to become wiser than the wise, or more learned than the learned." If anyone does things in violation of this Canon, let him be excommunicated for forty days. But if any layman chance to be experienced in discourse and modest in manner, he is not prohibited from answering and teaching in private those asking questions, as Zonaras states, and ch. 32 of Book VIII of the Apostolic Injunctions declare. For they shall be, it says, all taught of God: in which manner Apollos spoke, and taught the facts about the Lord, and in spite of the fact that he only knew the baptism of the Lord (Acts 28:25), and Aquilas and Priscilla, who taught the same Apollos the way of God more exactly.