The Letter was most likely written sometime between 85 and 90 AD, after the Gospel of John was written but before Revelation. It is not known to whom the letter is written.
It seems the main purpose of the letter was to reassure Christians in their faith and to counter false teachings. These two ideas go hand-in-hand. When John counters the false teachings he is reassuring the believers in their faith.
It is most likely that the false teachings were an early form of Gnosticism. Gnostics believed that knowledge is what brought true salvation. They believed in a secret knowledge that was built on the belief that the spiritual world (the invisible world) is pure and true. The physical world (the visible world) is evil and corrupt.
Of course, we know that this teaching is wrong because God created the world and declared that it was good. We believe that John is refuting this early form of Gnosticism because of his emphasis on the physical in 1:1 and 4:2-3. He also refutes the false teachers in 2:22.
It is also important to see John’s emphasis on “knowing” in the letter. The false teachers believe in a false-knowledge. John emphasizes the importance of knowing correct doctrine. In fact, John uses “to know” 13 times in this short letter.
Other key words/topics that are emphasized in the letter include sin, light, and love.
Perhaps the key verse is 5:13:
I write these things to you who believe in the name of the Son of God so that you have eternal life.
I pray as you read, study, meditate, and pray over this precious letter that you will be drawn closer to God by His Spirit and that you may know Christ with a desire to make Him known.