This is what John said:
1Jn 5:19 And we know that we are of God, and the whole world lieth in wickedness.
I suppose that some in his day would have considered him to be arrogant to consider that he and those whom he fellowshipped with were only of God and everyone else was lost.
1Co 1:10 Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment.
Rom 16:17 Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
Rom 16:18 For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple.
2Th 3:6 Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.
Gal 1:8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
Gal 1:9 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
What is the only book of the bible that contains the verbatim (word-for-word) preaching of the apostles to the lost telling them how to be saved and the lost obeying and getting saved? The book of Acts.
The simple fact of the matter is that those who are of God and are saved in obedience to God's truth, and live a life of loving obedience to his word, know that they are of God and are in truth. Anyone else that teaches something contrary to truth, is in error.