Early Church Father Barnabas refered to the cross in the shape of a letter T or Tau(greek)

 Chapter 9. The spiritual meaning of circumcision

He speaks moreover concerning our ears, how He has circumcised both them and our heart. The Lord says in the prophet, In the hearing of the ear they obeyed me. And again He says, By hearing, those shall hear who are afar off; they shall know what I have done. Isaiah 33:13 And, Be circumcised in your hearts, says the Lord. Jeremiah 4:4 And again He says, Hear, O Israel, for these things says the Lord your God. Jeremiah 7:2 And once more the Spirit of the Lord proclaims, Who is he that wishes to live for ever? By hearing let him hear the voice of my servant. And again He says, Hear, O heaven, and give ear, O earth, for God has spoken. Isaiah 1:2 These are in proof. And again He says, Hear the word of the Lord, you rulers of this people. Isaiah 1:10 And again He says, Hear, you children, the voice of one crying in the wilderness. Therefore He has circumcised our ears, that we might hear His word and believe, for the circumcision in which they trusted is abolished. For He declared that circumcision was not of the flesh, but they transgressed because an evil angel deluded them. He says to them, These things says the Lord your God— (here I find a new commandment)— Sow not among thorns, but circumcise yourselves to the Lord. And why speaks He thus: Circumcise the stubbornness of your heart, and harden not your neck? Deuteronomy 10:16 And again: Behold, says the Lord, all the nations are uncircumcised in the flesh, but this people are uncircumcised in heart. Jeremiah 9:25-26 But you will say, Yea, verily the people are circumcised for a seal. But so also is every Syrian and Arab, and all the priests of idols: are these then also within the bond of His covenant? Yea, the Egyptians also practise circumcision. Learn then, my children, concerning all things richly, that Abraham, the first who enjoined circumcision, looking forward in spirit to Jesus, practised that rite, having received the mysteries of the three letters. For [the Scripture] says, And Abraham circumcised ten, and eight, and three hundred men of his household. What, then, was the knowledge given to him in this? Learn the eighteen first, and then the three hundred. The ten and the eight are thus denoted— Ten by Ι, and Eight by Η . You have [the initials of the, name of] Jesus. And because the cross was to express the grace [of our redemption] by the letter Τ, he says also, Three Hundred. He signifies, therefore, Jesus by two letters, and the cross by one. He knows this, who has put within us the engrafted gift of His doctrine. No one has been admitted by me to a more excellent piece of knowledge than this, but I know that you are worthy.


Scholars date this this epistle  to between 70 A.D. and 130 A.D.  It also was referred to by many early church fathers. (ie. Clement of Alexandria)