I had always thought that the word "spoil" as used in the KJV meant that we would become like spoiled food--unusable. The KJV phrased the first part of verse 8 as: "Beware lest any man spoil you through philosophy...." But other translations render this as "take captive." That is when it dawned on me that this is the other meaning of spoil. This is more serious than becoming like spoiled food and worse than being taken as prisoner. Being taken as spoil indicates the reward of a pillager. Since pillaging is associated "especially with wartime efforts" (per the dictionary), that increases the gravity of the loss. Now, what has been taken captive, what has been taken as spoil, is not only our own loss of faith, but also refers to a loss that God and the congregation endure--losing a beloved fellow believer. The main pillager is Satan. He considers captive ones a war trophy and he doesn't hide the fact. In fact, he is boastful and arrogant, taunting Jehovah "Look what I got."
But how can philosophy take a person captive? During wartime, it has been documented that enemy propaganda was used to discourage POWs and others that were behind enemy lines. The propaganda was designed both to promote defection from the opposing side and to crush the hope that some had in their country's ability to liberate them. Likewise, Satan's philosophies are designed to promote defection from serving Jehovah and crush our hope that there really is a God that cares for mankind and wants to save us from Satan's dominion.
Yes, we are in a war and cannot afford to lose any of our dear friends. We each need to be on the lookout for the other's welfare, especially while on this battlefield.