I can safely say fear or unbelief comes by hearing, and hearing the words and lies of Satan. Fear and unbelief are just words that are contrary to faith in God's word; they are "tares among wheat". Words generate faith or fear.
In the most famous David and Goliath battle in Scripture, we see the power of words at display. Goliath, described as the champion of the Philistine, used words to instil fear into the hearts of military men.
And the Philistine said, I defy the armies of Israel this day; give me a man, that we may fight together. When Saul and all Israel heard those words of the Philistine, they were dismayed, and greatly afraid....And as he talked with them, behold, there came up the champion, the Philistine of Gath, Goliath by name, out of the armies of the Philistines, and spake according to the same words: and David heard them... And David spake to the men that stood by him, saying, What shall be done to the man that killeth this Philistine, and taketh away the reproach from Israel? for who is this uncircumcised Philistine, that he should defy the armies of the living God?" (1 Samuel 17:10-11, 23-26).Notice how Goliath used words to demoralise men -mighty men - of valour: “When Saul and all Israel heard those words of the Philistine, they were dismayed, and greatly afraid.” (vs11).
Now, bearing in mind Goliath's words, note that the same words that sent shivers down the spine of men, were the same words heard by David, a teenager. But his response was different. He took Goliath's words as an affront to God. “…for who is this uncircumcised Philistine, that he should defy the armies of the living God?”. He responded with faith in God:
Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the LORD of hosts, the God of the armies of Israel, whom thou hast defied." (vs 45)David dared Goliath based on his faith in God and his faithfulness: "David said moreover, The LORD that delivered me out of the paw of the lion, and out of the paw of the bear, he will deliver me out of the hand of this Philistine..."(vs37).
In any situation, make a choice to choose faith over fear and doubt. Take God at His word: "...hath he spoken, and shall he not make it good?" (Numbers 23:19).