Marked with a tau, i.e. with a cross. Tertullian says, “Haee 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).