John MacArthur, Spiritual Warfare : Who's After Whom ? de Our Sufficiency in Christ, 1991, p. 230.
Satan and his demons never, never act against God’s people without the permission of God. And when God gives them permission, He always uses their work to accomplish some divine purpose. Often, to exalt the power of God and prove the devotion of His followers, God permits Satan to work the hardest on the noblest servants of God.
John MacArthur, Spiritual Warfare: Who’s After Whom? from Our Sufficiency in Christ, 1991, p. 230.