The book of 1 John is the first of three letters that are included in the New Testament of the Bible that were written by John, son of Zebedee. John also authored the gospel that bears his name, and the book of Revelation. John was very close to Jesus during His ministry on earth and is referred to as, "the disciple whom Jesus loved".
This letter is most likely a circular, and although the intended recipients aren't named, it is clearly addressed to believers. It was written from Ephesus where John spent his later years, somewhere between a.d. 70 - 100. In this letter John speaks out against Gnosticism which was a dangerous distortion of the gospel that held that matter was evil, spirit was good, and the deeds of the flesh were of no consequence.