1 The Elder,

To the elect lady and her children, whom I love in truth, and not only I, but also all those who have known the truth, 2 because of the truth which abides in us and will be with us forever: 3 Grace, mercy, and peace will be with you from God the Father and from the Lord Jesus Christ, the Son of the Father, in truth and love.  I rejoiced greatly that I have found some of your children walking in truth, as we received commandment from the Father.
John focuses on the importance of truth in his writings and uses the word truth thirty-seven times in the New Testament. 
1. Whom I love in truth –  If we know and love the truth, we will love those who also the truth. 
2. Know the truth –  What is this truth- the word of the truth of the gospel (Col 1:5) 
3. The Son and the Father, in truth and love –  Apart from God’s truth and love, we can never really have grace, mercy and peace. 
4. Walking in truth –  Truth is not only important for its own sake, but also our walking in truth shows we are walking with the Lord.  
The word truth in the Greek language in verses 2 and 4 means the divine truth, gospel truth or the faith in practice of the true religion is called “truth” either as being true in itself and derived from the true God in opposition to the worship of false idols. 
 5 And now I plead with you, lady, not as though I wrote a new commandment to you, but that which we have had from the beginning (1 John 3:11): that we love one another. 
In John 13:34-35 (and other passages), Jesus gives us a new commandment: "Love one another as I have loved you." This selfless, Christ-like love is the defining mark of His followers. "By this, everyone will know that you are my disciples, if you love one another." Our love for one another is the visible evidence of our faith and obedience to Christ.
1 John 4: 7-11 God’s Love and Ours
7 Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. 8 Whoever does not love does not know God, because God is love. 9 This is how God showed his love among us: He sent his one and only Son into the world that we might live through him. 10 This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins. 11 Dear friends, since God so loved us, we also ought to love one another.

6 This is love, that we walk according to His commandments. This is the commandment, that as you have heard from the beginning, you should walk in it. 
John 14:15-18 Jesus Promises the Holy Spirit
15 “If you love me, keep my commands. 16 And I will ask the Father, and he will give you another advocate to help you and be with you forever— 17 the Spirit of truth. The world cannot accept him, because it neither sees him nor knows him. But you know him, for he lives with you and will be[a] in you. 18 I will not leave you as orphans; I will come to you.
1 John 2:5 
5 But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him.
1 John 2:24  Let Truth Abide in You
24 Therefore let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father.
1 John 5:3 
3 For this is the love of God, that we keep His commandments. And His commandments are not burdensome. 

7 For many deceivers have gone out into the world who do not confess Jesus Christ as coming in the flesh. This is a deceiver and an antichrist  
2 John 2:19 
19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.
2 John 2: 22 
22 Who is a liar but he who denies that Jesus is the Christ? He is antichrist who denies the Father and the Son.
1 John 3- 24 The Spirit of Truth and the Spirit of Error
24 Now he who keeps His commandments abides in Him, and He in him. And by this we know that He abides in us, by the Spirit whom He has given us.
1 John 4: 1-4  Love for God and One Another
4 Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. 2 By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, 3 and every spirit that does not confess [a]that Jesus [b]Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world.
4 You are of God, little children, and have overcome them, because He who is in you is greater than he who is in the world.

Conclude 
This shows us that what binds Christians together is not social compatibility or political compatibility or class compatibility the truth of the gospel of Jesus our Messiah and love for each other. When we do this, we are beings obedient to our Lord we say we love and serve.