Psalms 34:4 (NKJV) I sought the LORD, and He heard me, And delivered me from all my fears.
That verse is referring to what Jesus did when He came. He delivered us from everything that we could possibly fear. So it’s no wonder that when our minds become perfected in what Jesus did, it casts out ALL fear.
1 John 4:9, 18 (NKJV) In this the love of God was manifested toward us, that God has sent His only begotten Son into the world, that we might live through Him. 18 There is no fear in love [love = Jesus’ finished work] but perfect love casts out fear, because fear involves torment. But he who fears has not been made perfect in love.
Jesus already delivered us from all of our fears. And so, 1 John is just telling us that when your mind catches up to what Jesus actually did, there isn’t any more room for fear.