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The First Epistle of John is a book of the Bible New Testament, the fourth of the catholic or "general" epistles. It was written in Ephesus about 90-110 AD, apparently by the same author or authors who wrote the Gospel of John and the other two epistles of John. Not actually a letter, it is a sermon written to counter the heresy that Jesus did not come "in the flesh" but only as a spirit. It also defines how Christians are to discern true teachers: by their ethics, their proclamation of Jesus in the flesh, and by their love. (Summary from Wikipedia)