"Yea, though I walk through the valley of the shadow of death, I will fear no evil." The psalm does not pretend that evil and death do not exist. Terrible things happen, and they happen to good people as well as to bad people. Even the paths of righteousness lead through the valley of the shadow. Death lies ahead for all of us, saints and sinners alike, and for all the ones we love. The psalmist doesn't try to explain evil. He doesn't try to minimize evil. He simply says he will not fear evil. For all the power that evil has, it doesn't have the power to make him afraid.
And why? Here at the very center of the psalm comes the very center of the psalmist's faith. Suddenly he stops speaking about God as "he," because you don't speak that way when the person is right there with you. Suddenly he speaks to God instead of about him, and he speaks to him as "thou." "I will fear no evil," he says, "for thou art with me." That is the center of faith. Thou. That is where faith comes from.
- from Secrets in the Dark