John describes it as a spirit within people
No, he did not. Go back and re-read it.
You did a good job citing and quoting the four verses in which "antichrist" is mentioned. You did a good job of correctly articulating the criteria John provided. We might debate whether the criteria is exhaustive but for the sake of this op that may not be necessary. One error, however, was the assumption the antichrist is "
a spirit within people," and is possible only when "
a deceiving spirit within" exists in a person. That is reading into the text things not stated.
All a person needs to be an antichrist is sin.
This should be obvious because the deceiving spirit(s) does not have a deceiving spirit within making it be an antichrist. It does, however, have sin that does so. In other words, Satan did not need any additional spirit within him or to possess him for him to be an antichrist (assuming he was an antichrist). Evil spirits did possess some people throughout the gospels, but people do not need to be possessed by evil spirits to be evil. Sin is all that is needed. Sin is what corrupted the angels-turned-demons that then corrupt
some humans.
John makes it clear humans or angels can be antichrists, but he does not state human antichrists are antichrists only because they are possessed. Most who deny Christ are simply deluded. They do not need a spirit within them to delude them. They can delude themselves just fine on their own. I would venture to say sinful flesh does that far more often than evil spirits. Remember: fallen angels are finite, not infinite. They are creatures, not the Creator. They cannot be everywhere at once.
The word "spirit" is not always indicative of an extra-physical creature. Sometimes it simply means the attitude or character of a person. When we talk about things like "
team spirit" we are not saying there is an actual spiritual entity governing the team. Look at 1 John 4:6 again. All angels, all spiritual beings come from God simply because God and God alone is the Creator. But having their created origin by the hand/word of God does not mean the "come from" God in terms of their purpose. When John wrote of a "spirit of truth" and a "spirit of falsehood," he is not saying every rebellious spirit has another spirit dwelling within them that goes by the name "spirit of falsehood." If that were the case there would be many spirits of falsehood, not just one. There would be a spirit of falsehood indwelling and influencing every single rebellious angel. There would also be many more spirits of falsehood because there'd be one for every human denying Christ came from God as the Son of God. They would then, therefore, be the most numerous type of angel. Satan is not a denier because some other spirit makes him that way. Satan is a denier because he is dead and enslaved to sin.