Of the beliefs and practices whether generally accepted or publicly enjoined which are preserved in the Church some we possess derived from written teaching others we have received delivered to us “in a mystery” by the tradition of the apostles; and both of these in relation to true religion have the same force.
And these no one will gainsay;—no one, at all events, who is even moderately versed in the institutions of the Church. For were we to attempt to reject such customs as have no written authority, on the ground that the importance they possess is small, we should unintentionally injure the Gospel in its very vitals; or, rather, should make our public definition a mere phrase and nothing more.
For instance, to take the first and most general example, who is thence who has taught us in writing to sign with the sign of the cross those who have trusted in the name of our Lord Jesus Christ? What writing has taught us to turn to the East at the prayer? Which of the saints has left us in writing the words of the invocation at the displaying of the bread of the Eucharist and the cup of blessing? For we are not, as is well known, content with what the apostle or the Gospel has recorded, but both in preface and conclusion we add other words as being of great importance to the validity of the ministry, and these we derive from unwritten teaching.
Moreover we bless the water of baptism and the oil of the chrism, and besides this the catechumen who is being baptized. On what written authority do we do this? Is not our authority silent and mystical tradition? Nay, by what written word is the anointing of oil itself taught? And whence comes the custom of baptizing thrice? And as to the other customs of baptism from what Scripture do we derive the renunciation of Satan and his angels? Does not this come from that unpublished and secret teaching which our fathers guarded in a silence out of the reach of curious meddling and inquisitive investigation?
Well had they learnt the lesson that the awful dignity of the mysteries is best preserved by silence. What the uninitiated are not even allowed to look at was hardly likely to be publicly paraded about in written documents.
— St. Basil the Great, The Book of Saint Basil on the Spirit, Chapter XXVII