I Jn 2:3 says, “Hereby we do know that we know him.”  John is addressing the assurance of salvation.  Many people who have been saved have been told that they are assured of their salvation.  But many of those don’t know why they should be sure.  The first epistle of John tells us why.  John says, “Hereby we do know that we know him.”  We know that we know him because:

We keep his commandments [1 Jn 2:3-7, 5:2-3]. If a man says he knows Jesus and doesn’t keep his commandments he’s a liar [4:8; 5:10].  Before we were saved we didn’t even know his commandments, much less did we hope to keep them.  But after we were saved, we began to desire to please him and do those things that he wants us to do.  The first command is, “Believe on the name of Jesus Christ,” [1 Jn 3:23; 5:1].   His second command is to “Love one another,” [1 Jn 3:23, 4:21]

We love one another [1 Jn 3:14-15; 4:7, 4:16, 5:1].  We didn’t love Christians when we were living for the devil.  But after we were saved, we began to enjoy the fellowship of Christians and we began to love our brothers and sisters in Christ.  What a change.

We do righteousness [1 Jn 2:29].  We have the righteousness of Jesus Christ, which was imputed to us at our salvation [Rom 4:4-7].  Now we yield to his righteousness and he works through us to bring forth righteousness in our lives [Rom 6:13].  We work on the outside what God is working on the inside [Phil 2:12-3].  If you’re not keeping his commandments, loving the brethren and doing righteousness, then you aren’t sure of your salvation.  But he is sure.  We are known by God [1 Jn 3:19-21] even if our hearts condemn us [2 Tim 2:19, Jn 10:27-29].

We have his Spirit abiding in us [1 Jn 3:24; 4:13].  And with the Spirit of God dwelling in us, we have the witness in ourselves [1 Jn 5:10; Rom 8:16].

We have God’s record that he hath given us eternal life [1 Jn 5:11-13].  You can put your finger right on the words of God and know for sure that you are saved.  God doesn’t lie about such things.

We have an understanding that we are in Jesus [1 Jn 5:20].  Thus, we know that we have eternal life.  We are in Christ.  We are members of his body [Eph 5:30-32] and we are seated in heavenly places in him [Eph 2:6].  

Conclusion: John says these are the ways that you can be sure that you are saved.