John's gospel was written to move readers to faith in Jesus (John 20:31); 1 John was written to give believers the ability to find assurance of their salvation. According to Calvin:
As there ought to be a daily progress in faith, so he says that he wrote to those who had already believed, so that they might believe more firmly and with greater certainty, and thus enjoy a fuller confidence as to eternal life.
John lists several traits that should be evident in one claiming to be born again by the Spirit of God: They...
(1) desire fellowship with God and others "in Christ" (1 John 1:2-3; cf. Acts. 2:42-47; 1 Cor. 1:9; Heb. 10:25)
(2) have joy (1 John 1:3; cf. John 15:11; Phil. 1:4, 18, 25, 26; 2:2, 28; 3:1, 4:1, 4, 10; 1 Pet. 1:8)
(3) are sensitive to and honest about their sin (1 John 1:5-6, 8-10; Rom. 3:23; cf. Psa. 66:18)
(4) obey God's commands (1 John 2:3)
(5) love not the world (1 John 2:12-17; 5:4, 19)
(6) demonstrate purity in hope of Christ's return (1 John 3:2-3; Phil. 3:20-21; Tit. 2:11-13)
(7) have a decreasing pattern of sin in their lives (1 John 3:4-10)
(8) love others (1 John 3:10; cf. John 13:35; 15:13; 1 Thess. 4:9; 1 Pet. 1:22)
(9) have their prayers answered (1 John 3:22; 5:13-15)
(10) acknowledges Jesus came in the flesh (1 John 4:2-3; 1 John 2:22; 2 John 1:7 )
(11) suffered (ing) for Christ (1 John 4:13; cf. 1 Cor. 4:13; Phil. 1:28; 1 Pet. 2:20; 4:4; 5:10)
(12) are able to discern between truth and error (1 John 4:1-6; cf. 1 John 2:21; 2 Tim. 3:16; 1 Thess. 5:21; cf. Deut. 29:29)
(13) experience the presence and ministry of the Holy Spirit (1 John 4:13; cf. 1 John 2:27; 1 Cor. 2:10, 12; Gal. 4:6; 5:22-23; Rom. 8:16)
(14) demonstrate faith (1 John 5:1)
(15) accept the testimony of God concerning his Son (1 John 5:9-12; John 14:6)
(16) keep themselves from idols (1 John 5:21).
While many of the above traits overlap one another, this is not a smorgasbord! The true Christian will not consider if he can get away with taking 1, 7, and 10 above and leave the rest at the Lord's Table. No, as a new creation he desires all that has been freely given him in Christ, that he fall behind in no good thing. He looks not for excuses, but fulness (Matt. 5:6), which he has found and will continue to find at the foot of the cross.
"Examine yourselves, to see whether you are in the faith. Test yourselves" (2 Cor. 13:5). Albeit, Christians are not perfect in all these characteristics at rebirth (1 John 1:8-10); they nonetheless have these traits. Even though believers are not like Christ in terms of the completeness of their obedience, they are like him in their basic orientation and stand out, as Christ did, in contrast with the world at large (John 5:44; 17:16). Since believers are new creatures (2 Cor. 5:17), endowed with all things pertaining to life and godliness (2 Pet. 1:3), they will let their light so shine (Matt. 5:16; cf. Phil. 3:8; 1 Thess. 2:12; 2 Pet. 2:20). See "Make Your Calling and Election Sure - 2 Peter 1:10" below. See WCF 18.1; 18.3; WLC 80, 172; WSC 36.
so that you will know. See
WCF 18.1, 3;
WLC 80; 172;
WSC 36. Repeatedly, John has referenced this purpose for writing—to assure his readers that they know God. Often, he employs verbs for knowing in this short letter: oida (2:11, 20-21, 29; 3:2, 5, 14-15; 5:13, 15, 18-20) and ginōskō (2:3-5, 13-14, 18, 29; 3:1, 6, 16, 19-21, 24; 4:2, 6-8, 13, 16: 5:2, 20). Uniquely among the evangelists, John says Christ's followers will not only possess eternal life in the future, they also experience it now, because they already enjoy fellowship with God (see notes on 1:3-4; Jn. 17:3). This assurance of true knowledge arises from a true confession of Jesus as the Messiah, obeying him and loving your brothers and sisters in your words and deeds.