Why does God allow the Devil so much free reign on the Earth?

I am a devout Christian. I have gone to church for most of my life. I do not read the Bible very much, but I know many of the lessons that it tries to teach us. One question that I can never figure out an answer to is this one. Why did God not simply destroy Satan when he rebelled? Why is Satan allowed so much power on the Earth? I have read some of the Book of Job and heard much about it, so I know the lessons of following God and being rewarded in life and in the next life. Is God trying to give Satan, who God made with free will, a chance to repent? Is the Devil just a means of testing the faith and morality of humanity? Is the Devil just proof that God gave us free will? I reject the Devil and his ways as much as I can. As a Christian, I wonder why God allows misfortune to come upon those that have already proven to be moral. Why does God allow Christians, his people, to suffer when they follow him and his word? I do not understand. By the way, if you are simply going to answer me with God doesn't exist or I am an idiot for believing in something, then save us both time and energy and don't answer.