Many Christians don't realize John probably wrote this letter after his experience on the Island of Patmos, just off the coast of modern Turkey, where he had written the book of Revelation. It is generally believed after he wrote Revelation he was released from imprisonment by the Romans, returned - probably to Ephesus - and finally wrote 1 John at the very end of his life.
This presses urgency into John's words. John has no interest in just comparing religions or philosophies. He's not conducting a survey or parsing his phrases around cultural political correctness. He's talking about a revealed announcement that Christ Jesus that alone can get you and me ready for the Second Coming of God the Son. This letter is a summary of what must be kept in place - what must be at the very center of our hearts - if we're going to be successful in following our Lord through all that is to come.
John lived long enough to know how spiritual passions cool and wane. Not everyone sees the things he saw in his vision on Patmos. We're so prone to distractions. Secondary things become primary and primary things become secondary. This letter is what John, knowing what he knows about the future coming of Christ's kingdom, wants these Christians to keep boiling hot on the front burner of their lives.