The fierce spiritual warfare. This is not confined to the mission field, but certainly the battle is fiercest in that arena. Satan will not suffer his kingdom to be invaded or his authority challenged by the Christian missionary. Sooner or later the missionary will feel the full fury of his wrath. Satan may attack his mind, body, or soul, but attack he will, and the missionary will need the protection of the whole armor of God.
Herbert Kane, Life and Work on the Mision Field