There is a famous Jewish teaching that if you want to find out the true meaning of something, find the first time the word appears in the Bible. The first time a form of the word 'Satan' appears is in Numbers 22:22 where an angel of G-d stood in front of Bilam and his donkey to obstruct them, to block their path. The word "l'satan", therefore, means to be an obstacle, an obstruction, a roadblock, an adversary, etc.
The purpose of Satan and the Satanic force in the world is to obstruct our spiritual progress. Life is supposed to be a path to G-d. If the journey were a breeze, with no challenges, there would be little virtue in succeeding. For example, G-d tells us not to commit adultery. If there were no strong sexual impulse, and if people of the opposite sex looked like sacks of potatoes, there wouldn't be much virtue in being moral. G-d asks us to return the lost objects of people that we may happen to find. If we had no lust for money, then there would be little virtue in returning someone's wallet. The rabbis teach in the Talmud that Satan basically resides inside of us, and it's called the Yetzer Hara (Inclination Toward Evil). The rabbis say that this Satanic force is the greatest blessing that G-d ever gave us! Satan is the loyal opposition. It helps us build our spiritual muscles by giving us resistance to our spiritual progress. If you go to a gym, and lift weights that weigh 6 ounces, you will never build any muscle. The more weight you add, the more resistance, the more muscle you will build. The same is true spiritually. If we didn't have this Satanic force in the world opposing our progress, there would be no opportunity to build our spiritual muscles...
The greatest chess player in modern times was arguably Garry Kasparov. His greatest opponent was Anatoly Karpov. They were bitter enemies. But without Karpov, there would be no Kasparov. Having such strong opposition brought out the best in him. It made him the great chess player that he was. If he only had weak competition, he would never have grown into a great chess player. So, his strongest enemy was the greatest blessing to him! The Talmud says that the more righteous someone is, the greater is their Yetzer Hara. G-d needs to test him even more since he's reached such a high level!
What a totally different understanding of Satan than the Christian one! To them, Satan is not playing on G-d's team. He's the leader of a team that has rebelled against G-d. From a Jewish point of view, Satan is on G-d's team! And even though he tries his hardest to tempt us and detour us, he is ultimately rooting for us to overcome him! He's simply doing his job!
Rabbi Michael Skobac