The book of First John was written by Jesus‘ disciple John, who also wrote the John. As such there are many similarities between the two books. He is also the author of Second John, Third John, and Revelation.
In reading this book, look for the reoccurring theme of love, which is mentioned twenty-four times. Other repeated words to pay attention to are hate (five times), truth (eleven times), spirit (eight times), liar (five times), and fellowship (three times).