The Good God can't keep out evil because he's not fully in control; the Evil God (Satan) currently has power over the earth. This is probably the closet view of the majority of Christians, but Christianity has always rejected this notion, and for good reason. According to the bible, only one God is all-powerful, and all evil forces work only because he allows them to.
Gnosticism is tempting because it seems to solve the Problem of Evil, but it does so at the expense of God's power. It draws a Satan almost co-equal with God, and along the way introduces several other concepts which have rippling effects: the idea that flesh is bad and spirit is good, the concept of the physical creation being corrupt and good only for destruction, and the concept that our whole society and world is going downhill. Christianity has always rejected all of these ideas as well, and yet they've always managed to creep in. They creep in because gnosticism seems to give us an answer.
A common answer to the Problem of Evil is to simply say that evil is Satan's fault. But this is like saying that you were babysitting your kids, and the neighborhood bully decided to come in and beat them up, so you just stood aside and watched it happen. Then, when people accused you of being a bad parent, you pointed out that it wasn't YOU who beat the kids up, it was the bully.
If God is not powerful enough to stop Satan, then gnosticism is true, and God's power is diminished. But if God is powerful enough to stop things, and yet just sits by and watches Satan work, then he is not really all-loving. Either God is not really in control, or God is a negligent parent.
Probably the most popular current answer to the Problem of Evil is to say that all evil is the result of Adam's sin. But this is not really an answer.
Most of these arguments serve only to disguise the blame, not actually to deal with it. All of these answers do one of two things: they give up some of God's power, making him less powerful against Satan, or they reduce his love for mankind. And most of them lead us down the road of gnosticism, and all the negative trappings that come with that.
Remember, the bible stands in stark opposition to all of these trains of thought. All of creation is "very good", all of it functions according to God's commands, Jesus came as a physical human, and Satan is ultimately under God's control.
...to be continued...
