E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Marked with a tau, i.e. with a cross. Tertullian says, Hæc est litera Græcorum , nostra autem T, species crucis. And Cyprian tells us that the sign of the cross on the forehead is the mark of salvation.
This reward (Ezek. ix. 4) is for those whose foreheads are marked with Tau.Bp. Andrews Sermons (Luke xvii. 32).