On the Forehead

Goliath is dressed in metal: Bronze helmet, bronze scale armor, bronze greaves. His javelin is bronze, the head of his spear is iron. 

The original metal outfit in the Bible is the breastplate of the high priest, with its gold thread woven with other threads. 

With his scale armor, Goliath is a serpent. With his metal clothing, he is a serpent-priest, blaspheming rather than praising Yahweh.

One thing he lacks: On his forehead, the high priest wore a “stone” of gold (cf. Zechariah 3:9), inscribed with “Holy to Yahweh.”

Goliath has no forehead stone, until David provides it.