You have asked about this sentence from 1 John 4:4: "Little children, you are of God, and have overcome them; for he who is in you is greater than he who is in the world."
This has proved to be an important and useful affirmation for many people. Jesus taught that we must become as little children—that is, reexperience the sense of Oneness with Spirit and infinite Love that children innately feel—in order to move forward to Christ consciousness. When we are clearly connected with, and expressing, that Christ consciousness—our Oneness with all that God is—then we have already overcome all the contrary, negative, fear-based voices we may still encounter. The battle may still seem to be waging, but the war is won. When we know ourselves to be the Christ, nothing can prevail against us, for the Christ is God—greater than any contrary illusion.