There are some things that Christians believe about the devil. The Bible teaches that the devil was an angel, that led a rebellion, and with him one third of the angels became fallen.
The question that naturally arises is, how do you stage a rebellion attempting to overthrow an all-powerful and all-knowing God? God will know what you are up to, and absolutely nothing you do can harm Him. So how is rebellion even possible? Why didn’t God stop it before it ever got started?
As how rebellion is possible, the devil convinced himself that he could successfully rebel, being blinded by his own arrogance and vanity. He lied to himself to believe he could succeed.
But answering why God allowed it, has to come down to one reason. The devil’s rebellion served God’s purposes. Now for some people this is a hard pill to swallow. For them, thinking God allowed sin, for any reason, is reprehensible. I prefer to look at it that God is powerful enough, that every sin, is used by God to bring about positive results.
I believe God’s purpose, His positive result, is to create humanity as truly and completely free moral agents. We have been given the power of choice. However, the power to choose is meaningless without choices to choose from. Similarly, if all of the choices are right choices, then we are not truly free.
The devil creates for humanity the alternative to choosing right. Without this choice we would not be truly free, and therefore not truly human. The purpose of our humanity is the ability to choose God, choosing to love and serve Him instead of self. This human free will is what makes us the crown of creation, and without it we would be nothing more than fancy automaton.
The next question that arises is, does that mean God is responsible for sin? The answer is no. God creates angels and humans with a true power of choice, and with this power comes the responsibility for one’s own choices. God knew the wrong choices that would be made, but that does not make Him responsible for our sins. We choose them and we are responsible for them.
To help illustrate this point, think about a couple bringing a new baby home to a two year old. They can reasonably assume the older child will at some point mistreat the younger. So does that mean they forcibly separate the two and never let them meet? Of course not, to do so would be an unspeakable cruelty. These two are born to be siblings and to love each other. So over their growing up years the parents know there will be errors, but seek to teach them to love and grow together into healthy, non-murderous, siblings.
Similarly God is not responsible for our sins. He is simply allowing the situation which gives us the possibility of becoming fully what God has created us to be. He made us to be the crowning jewel of His creation, the ones who choose to love Him.