The way we may be sure that we know him is to keep his commandments. Whoever says, "I know him," but does not keep his commandments is a liar, and the truth is not in him.
But whoever keeps his word, the love of God is truly perfected in him. This is the way we may know that we are in union with him: whoever claims to abide in him ought to live (just) as he lived.
1John 2:3-6
If someone who has worldly means sees a brother in need and refuses him compassion, how can the love of God remain in him? Children, let us love not in word or speech but in deed and truth.
(Now) this is how we shall know that we belong to the truth and reassure our hearts before him in whatever our hearts condemn, for God is greater than our hearts and knows everything.
Beloved, if (our) hearts do not condemn us, we have confidence in God and receive from him whatever we ask, because we keep his commandments and do what pleases him.
And his commandment is this: we should believe in the name of his Son, Jesus Christ, and love one another just as he commanded us. Those who keep his commandments remain in him, and he in them, and the way we know that he remains in us is from the Spirit that he gave us.
1 John 3:17-24
If anyone says, "I love God," but hates his brother, he is a liar; for whoever does not love a brother whom he has seen cannot love God whom he has not seen. This is the commandment we have from him: whoever loves God must also love his brother.
1 John 4:20-21
We also know that the Son of God has come and has given us discernment to know the one who is true. And we are in the one who is true, in his Son Jesus Christ. He is the true God and eternal life.
1 John 5:20