John begins chapter four with the counsel, “Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world” (4:1). John is concerned about those who provide false testimonies and teaching about Jesus. There seems to be a broad principle in play here, that is, be very careful to examine everything in the light of God's Word and test whether or not what we are seeing and hearing is truly from God. That's a good piece of advice.
The rest of the letter is devoted, largely, to the notion of “love,” love toward God and love toward one another. John writes, “In this is love, not that we have loved God but that he loved us and sent his Son to be the propitiation for our sins. Beloved, if God so loved us, we also ought to love one another”(4:10,11). One of the great identifying traits of a true believer! Having been loved by so great a love, the child of God just radiates love!
Where charity and love prevail
There God is ever found.
Brought here together by Christ’s love
By love are we thus bound. --Ubi caritas, Translator: Omer Westendorf (1961)