You are right in that God desires us to grow into spiritual men and women, but I dare say that any of us could ever have nothing negative effect us, and stand in perfect faith. This is a goal which God sets for us, in our walk to Christlikeness, but it is not obtainable by humans. As humans we will never be capable of perfect faith...if we were, we would not know the world as it is.God would have us grow into spiritual men and women of God. Totally filled with the Holy Spirit, to a point, nothing negative effects us, but we stand in perfect faith and conviction for God, producing that "fruit" in our lives.
Where did God give Satan permission? I only see where the boundaries were set.Know that Scriptures prove, Satan could never do anything to anyone without God's permission first- (Book of Job).