Welcome to 1 John!
Below is helpful information I found in Fee and Stuarts Book, “How to Read the Bible Book by Book:“
Orienting Data for 1 John
Content: a treatise that offers assurance to some specific believers, encouraging their loyalty to Christian faith and practice—in response to some false prophets who have left the community.
Author: the same author who wrote 2 and 3 John, who there calls himself “the elder”; a solid historical tradition equated him with the apostle John.
Date: unknown; probably toward the end of the first Christian century (the late 80s, early 90s).
Recipients: a Christian community (or communities) well known to the author (whom he calls “dear children” and “dear friends”; the false prophets defected “from us,” 2:19); it has traditionally been thought to be located in or around Ephesus.
Occasion: the defection of the false prophets and their followers, who have called into question the orthodoxy—both teaching and practice—of those who have remained loyal to what goes back to “the beginning.”
Emphases: that Jesus who came in the flesh is the Son of God; that Jesus showed God’s love for us through his incarnation and crucifixion; that true believers love one another as God loved them in Christ; that God’s children do not habitually sin, but when we do sin, we receive forgiveness; that believers can have full confidence in the God who loves them; that by trusting in Christ we now have eternal life.
You can experience some real ambivalence in reading 1 John. On the one hand, John’s writing style is very simple, with a very limited and basic vocabulary (so much so that this is usually the first book beginning Greek students learn to read). It also has a large number of memorable—as well as some profound—moments. On the other hand, you may experience real difficulty trying to follow John’s train of thought. Not only is it hard at times to see how some ideas connect with others, but certain, obviously significant, themes are repeated several times along the way.
Although, like most of Paul’s letters, the aim of 1 John is to persuade, it nonetheless does not come in the form of a letter (notice that there is no salutation or final greeting). Most likely this is because John is writing to communities where he has direct oversight. What he writes includes teaching that “you have heard from the beginning” (2:24) about “the Word of life” who “was from the beginning” (1:1; cf. 2:13).
The primary concerns are three: the Incarnation; love for the brothers and sisters, especially those in need; and the relationship between sin and being God’s children. The first two of these are the more urgent and are expressed together in 3:23: “This is his command: to believe in the name of his Son, Jesus Christ, and to love one another.”
Specific Advice for Reading 1 John
As you read, be especially on the lookout for what John says about the false prophets, since they are the key to everything. Note that they have recently left the community (2:19), but only after trying to lead the whole church astray (2:26; 3:7; 4:1). These prophets apparently considered their teaching to come from the Spirit (cf. 4:1), which is why John urges that the believers’ own anointing of the Spirit is sufficient for them (2:20, 27; 3:24). Indeed, in a marvelous wordplay on the language of “anointing” (chrisma), he calls the false prophets “antichrists” (anti-christos = against the Anointed One). There has been much speculation about who these false prophets are, or what heresy they represent, but in the end, these things cannot be known for certain, except that they deny the Incarnation, fail to love those in need, and (perhaps) argue that they are sinless.