In Judaism we do not see it as there being a conflict between good and evil. Some ancient religions believed that there are two forces in the universe, one good and one evil, and that they are constantly warring with each other. This was common to Mithraism, Zoroastrianism, Christianity, and later to Manichaeism. Christianity, however, made the devil less powerful than G-d, but still made him a rebel against G-d.
Judaism sees it differently, and we have always seen it differently. Satan is not a rebellious angel. G-d created both good and evil. The Bible says so, in Isaiah 45:7.
What is evil? Not fulfilling G-d's will.
G-d created the universe because G-d wanted to do good. So there had to be people to receive that good.
But G-d does not want to just give away good as a present. G-d wants people to appreciate it. Something you get for free you do not appreciate. And in fact, if you got something amazingly good for free, and you were allowed to enjoy it for all eternity, you would be embarrassed by it. You didn't work for it, you don't deserve it.
So G-d decided that people would have to work for it, and receive the ultimate goodness as a reward for work.
What is that work? Well, G-d created the Evil Inclination, the angel called Satan, whose job it is to tempt us to do evil. If we ignore the Evil Inclination, then we get closer to G-d, and become more holy. By doing so, we merit the reward of the ultimate goodness. G-d also gave us Commandments, and the Tempting Angel tempts us to find reasons not to keep those Commandments. By ignoring the temptations, and fulfilling G-d's Commandments, we become more spiritual, and our souls gain more power over ourselves.
So we see our lives in this world as an opportunity. We have been granted the glorious opportunity to attain holiness and ultimate goodness.
So, in the future there will come a time when the universe as we know it will come to an end, and the World To Come will begin. Then we will begin to get our eternal reward -- the ultimate goodness!
This world is here merely for us to do good things in, and the Next World is for the reward. And in that world we will continue to grow in holiness.
So it is not that there is an ultimate struggle of good versus evil. Within each of us there are many struggles. We want to do the right thing, but we desire to do the wrong thing. We have the free choice to choose. We can do either the right thing or the wrong thing. It is completely up to us.
When we let our Good Inclination prevail about one thing, we become stronger in that thing. The more we continue to let the good prevail, the more it becomes easier to do the right thing in that area.
Say, for example, we have a desire to tell harmful gossip about someone. If we hold ourselves back from doing it, and continue to hold ourselves back whenever we feel the temptation, it becomes easier and easier to stop gossiping. Conversely, the more we gossip, the harder it is to stop. And that's the way it is with every individual characteristic trait.
But when we perfect one characteristic trait, it does not always help with another. For example, if we work on becoming humble, sometimes that will help us in other areas. A humble person will be less likely to get angry at someone else. A humble person might not tell gossip either. But a humble person might still lie, or sleep late when he or she knows that there is a chore or other responsibility to deal with. Someone who has learned never to gossip might still steal, for example. So each trait usually has to be worked on by itself, but something like humility helps with most of the others.
And possibly the greatest temptation is in the area of having faith in G-d. Developing our faith helps us in many areas.
So, there is no ultimate struggle of good versus evil. There is only the struggle within each of us. The Evil Inclination, also known as Satan, is doing what G-d has commanded him to do. He is giving us temptations, because by fighting against our wrong desires we are working to gain the ultimate goodness, and that is what G-d wants us to do
Mordechai Housman