By This We Know – Letters of John
John now begins a new section with an encouragement on assurance for true believers - that you can know God by obeying His commands.
1 John 2:3-11
3 And by this we know that we have come to know him, if we keep his commandments. 4 Whoever says “I know him” but does not keep his commandments is a liar, and the truth is not in him, 5 but whoever keeps his word, in him truly the love of God is perfected. By this we may know that we are in him: 6 whoever says he abides in him ought to walk in the same way in which he walked.
The New Commandment
7 Beloved, I am writing you no new commandment, but an old commandment that you had from the beginning. The old commandment is the word that you have heard. 8 At the same time, it is a new commandment that I am writing to you, which is true in him and in you, because  the darkness is passing away and the true light is already shining. 9 Whoever says he is in the light and hates his brother is still in darkness. 10 Whoever loves his brother abides in the light, and in him  there is no cause for stumbling. 11 But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes. (ESV)