The sign of the Tau, the last letter of the Hebrew alphabet, held a special place in the heart of St. Francis of Assisi.
In the year 1215 AD, five years after he and his first band of followers had travelled to Rome to seek the approbation of their new Rule from Pope Innocent III, the Pope convened the Fourth Lateran Council. The Pope called upon all of those present to mark themselves and all Christians within their ministry with the sign of the Tau, a symbol of penance and of the Cross. The Pope went on to affirm that the shape of the Tau indicated a Cross and as such would be symbolically borne upon the heads of all those who manifested its radiance in their own lives. The Pope then exhorted all present to be champions of the Tau and of the Cross.
The Pope’s request seemed to be the perfect message for St. Francis and it was most likely at this time that he chose the Tau as the symbol of his penitential life and as the resume of all his preaching. Therefore, the Tau became for St. Francis a very sacred symbol, a constant reminder to him of the Cross of Christ and of his need to be continually transformed through a penitential life into the image of the suffering and crucified Christ. For St. Francis, the symbol was rich and full of meaning, signifying something very dear to him. He sought to embody its significance in his own life and in the lives of all his followers.
Taken from: The Sign of the Tau A symbol of Franciscan Penance By Father Dominic F. Scotto, TOR