Edgar Conrad’s monograph on the “fear not” passages of the Old Testament concludes that they “represented stereotypical language used to encourage a warrior before battle.”
In Isaiah, there are two such “War Oracles” addressed to kings: Ahaz in chapter 7 and Hezekiah in chapter 37. But Conrad also claims that there are “War Oracles” addressed to the entire nation: “Yahweh addresses the community directly with War Oracles (10.24-27; 41.8-13; 41.14-16; 43.1-4; 43.5-6; 44.1-5). The use of the language stereotypical of royalty suggests that Yahweh is addressing the community as a king.”
Conrad’s study illumines Jesus’ constant refrain, “Fear Not,” showing that Jesus, like Yahweh, is assuring His disciples in the face of the quasi-military threats that they will face in their ministry (principalities and powers), and that He is treating them as a group of kings heading out to battle.