Even the belief in Satan, and the hint of dualism this entails, has caused Christian doctrine to distance itself from the existence of a Devil. Besides which, problems are still raised as to why God would create the Devil in the first place. If we are to maintain a belief in a theistic God, the understanding of his attributes need to be redefined. The 'Theistic God' exists. Man has been given free will by God.
Evil does exist.
God does have the power to stop us from doing evil, but if he did so he would be taking away our freedom to choose. By 'all-powerful', we do not mean that God can do literally anything imaginable. God, like all agents, has a nature, and so omnipotence must be understood in terms of God's nature. Aquinas argued that God cannot commit evil, because evil falls short of God's nature, which is goodness. One possible response is that the future is not yet known, even for God.