The Greek letter tau is one of the most ancient symbols known to the Church: its etymological predecessor – the Hebrew letter taw meaning truth – was already spoken of in the Book of Ezekiel as a symbol.
“And the LORD said to him: Pass through the city, through the midst of Jerusalem, and mark an X [taw] on the foreheads of those who grieve and lament over all the abominations practiced within it.” – Ezekiel 9:4
The Greek letter tau in its uppercase form looks exactly like the uppercase letter T from our alphabet, the Latin alphabet. Christians in the earliest days of the Church adopted the Greek letter tau as a symbol for the Crucifixion Cross because its uppercase form visually resembled it.
One the earliest recorded examples of the tau cross comes from the Epistle of Barnabas, written sometime between 70 – 132 AD. As tau was also the Greek numeral for 300, the Epistle gives an allegorical interpretation of the number 318 (TIH in Greek) from the Book of Genesis 14:14, including a reference to the tau cross.
“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.”
Saint Clement of Alexandria gives a similar interpretation of what he calls “the Lord’s sign” in his Stromata, also known as Miscellanies, written from 198 – 203 AD.
“They say, then, that the character representing 300 is, as to shape, the type of the Lord’s sign, and that the I and the E indicate the Saviour’s name.”
Tertullian does the same in his Adversus Marcionem saying the Greek tau looks like that Latin T which looks the True Cross.
“Ipsa est enim littera Graecorum Tau, nostra autem T, species crucis”
The tau cross is seen again as part of the staurogram in many early Christian papyrus manuscripts. In these manuscripts, the Greek word σταυρὸν – “stauros” – referring to the Crucifixion Cross was instead replaced with a ligature of tau (T) and rho (P) making the staurogram: ⳨
The tau of the staurogram represented the Cross, and the loop of the rho represented the head of Christ. In effect, the staurogram was a pictographic representation of the Crucifixion, making it perhaps one of the earliest of Christian images of Christ on the Cross.
“The staurogram constitutes a Christian artistic emphasis on the cross within the earliest textual tradition, in one of the earliest Christian artifacts we have, text and art are combined to emphasize ‘Christus crucifixus.'”
As an interesting aside, the mineral Staurolite takes its name to the fact it forms in cross shapes: