By this we know the Spirit of truth and the spirit of deception.
John writes to his agapētοi (dear ones), and cautions them:
Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
-- 1 John 4:1 (KJV)
Having identified the need to test the spirits, John goes on to enlighten them as to how they might do so:
2Hereby you know the Spirit of God.
-- 1 John 4:2-3 (KJV)
- Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
- 3And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
John informs his agapētοi that there are two spirits to be aware of: the Spirit of God, which moves people to acknowledge that Jesus Christ has come in the flesh, and the spirit of Antichrist that is now in the world, which would move people to deny that Jesus Christ has come in the flesh.
4Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world.
-- 1 John 4:4 (KJV)
Never mind teknia (little children), you have overcome those who are not moved by the Spirit of God, because the Spirit of God that is in you is greater than the spirit of Antichrist that is in the world.
5They are of the world: therefore speak they of the world, and the world heareth them. 6We are of God:
Hereby know we the spirit of truth, and the spirit of error.
- he that knoweth God heareth us;
- he that is not of God heareth not us.
-- 1 John 4:5-6 (KJV)
The conversation of those who are of the world is confined to worldly things, which the world longs to hear. On the other hand, we are of God and our conversation includes passing on the knowledge of the love of God that has been manifest to mankind through Jesus Christ. Those who want to know God will be able to hear such things, but those who don't, won't.
The love that is "of God"
7Beloved, let us love one another: for love is of God; and every one that loveth is born of God, and knoweth God. 8He that loveth not knoweth not God; for God is love.
-- 1 John 4:7-8 (KJV)
In verses 9-13, John details what he means by love that is "of God". It:
was demonstrated by God in the sending of His only Son so that those who know Him (the Father) might live through him (the Son) (v. 9)
is not of men, but is of the Father who sent His only Son to be an atonement for their sins (v. 10)
is expected to be emulated by those who would claim to know God (v. 11)
is the only means by which the Father's presence is sensible to those who are "of God", and once it is, can then be perfected in them (v.12,13)
And we have seen and do testify that the Father sent the Son to be the Savior of the world.
-- 1 John 4:14 (KJV)
John, here, attests to the truth of what he reports, having been witness to the unfolding the events surrounding the manifestation of the love of the Father through the Son, his Saviour, Jesus Christ.
John is not just talking about the general notion of love, but love that is "of God", the ideal towards which one who would claim knowledge of God, must strive -- love that is only possible through the presence of the Spirit of God.
A question that John might ask those to whom the OP refers, "Does your love extend as far as telling those you love, of the love "of God" as it was manifest in the life, death, and resurrection of His only Son, Jesus Christ?"
John started out by identifying for his readers two spirits that would move the hearts of men. One can logically conclude that they are either moved by the Spirit of God, or they are not. Not being moved by the Spirit of God to advertise the love of God that is manifest in the coming of His Son, one could then logically determine whether or not they are being moved by the spirit of Antichrist, by asking, "What am I moved to say to those I love about Jesus Christ? Did he come in the flesh, or did he not?"
For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
-- Hebrews 4:12 (KJV)
No one who truly reads the word of God can come away unchallenged.