St. Basil the Great (329-379) said:
Moreover we worship even the image of the precious and life-giving Cross, although made of another tree, not honouring the tree (God forbid) but the image as a symbol of Christ. For He said to His disciples, admonishing them, Then shall appear the sign of the Son of Man in Heaven, meaning the Cross. And so also the angel of the resurrection said to the woman, Ye seek Jesus of Nazareth which was crucified. And the Apostle said, We preach Christ crucified. For there are many Christs and many Jesuses, but one crucified. He does not say speared but crucified. It behoves us, then, to worship the sign of Christ. For wherever the sign may be, there also will He be. But it does not behove us to worship the material of which the image of the Cross is composed, even though it be gold or precious stones, after it is destroyed, if that should happen. Everything, therefore, that is dedicated to God we worship, conferring the adoration on Him.
The tree of life which was planted by God in Paradise pre-figured this precious Cross. For since death was by a tree, it was fitting that life and resurrection should be bestowed by a tree. Jacob, when He worshipped the top of Joseph's staff, was the first to image the Cross, and when he blessed his sons with crossed hands he made most clearly the sign of the cross. Likewise also did Moses' rod, when it smote the sea in the figure of the cross and saved Israel, while it overwhelmed Pharaoh in the depths; likewise also the hands stretched out crosswise and routing Amalek; and the bitter water made sweet by a tree, and the rock rent and pouring forth streams of water, and the rod that meant for Aaron the dignity of the high priesthood: and the serpent lifted in triumph on a tree as though it were dead, the tree bringing salvation to those who in faith saw their enemy dead, just as Christ was nailed to the tree in the flesh of sin which yet knew no sin. The mighty Moses cried, You will see your life hanging on the tree before your eyes, and Isaiah likewise, I have spread out my hands all the day unto a faithless and rebellious people. But may we who worship this obtain a part in Christ the crucified. Amen (St. Basil the Great, An Exact Exposition of the Orthodox Faith, Book 4, Chapter 9).
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 m 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 tim 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? (St. Basil the Great, On the Holy Spirit).