Isa. lxvi. 19, a passage interpreted by the Fathers of the sign of the Cross.  Eusebius (Demonstr. Evang. vi. 25):  “Who, on seeing that all who have believed in Christ use as a seal the symbol of salvation, would not reasonably be astonished at hearing the Lord’s saying of old time, And they shall come, and see My glory, and I will leave a sign upon them?”  Cf. Cat. iv. 14; xiii. 36.