It took you awhile. You are very wordy on a wonderfully simple matter. Why did you not simply cite 1 John 4:2?
There ya go. Using a prooftext that doesn't say what you say it means. Where does 1 John 4:2 say, "This is how you recognize a Christian?" No. It says "This is how you recognize the Spirit of God." The context is not at all about testing people, and for you to say it is constitutes an outright distortion. Read 1 John 4:1: "do not believe every SPIRIT," and goes on to advise to "test the spirits to see if they are from God."
If you still don't know realize what John is talking about, then I advise you to look at the passage to Timothy where Paul talks about "the doctrine of demons." (Yes, spirits have their counterfeit gospel & counterfeit doctrines)
As for why this passage is in 1 John, it's real simple. Every NT epistle was both eternal truth but was also written for a specific, local situation. Now what was John the disciple dealing with in churches at the time he wrote 1 John? Answer? Gnosticism, which taught that Jesus did not come in the flesh.
The one, eternal gospel was always customized to some degree in these epistles. Not every epistle addressed sexual immorality, for instance. But certainly the Corinthians needed to be reminded that the body is a temple of the Holy Spirit.
Jesus says outright that a lot of folks who believe the proper things aren't going to make it to the kingdom. (See Matthew 25 for instance). He adds in the previous chapter, "but he who stands firm to the end will be saved." (Matt 24:13). That, and Hebrews 6, implies that many people who believe 1 John 4:2 may not make it into His kingdom.
Bottom line: Do you or do you not believe that 2 Cor 11:4 teaches there are different Jesuses, different spirits, different gospels?: "For if someone comes to you and preaches a Jesus other than the Jesus we preached, or if you receive a different spirit from the one you received, or a different gospel from the one you accepted..."