It is surprising to note that this phrase "hereby know" occurs eight times in the little epistle of 1 John. Each of these listed below is given as a means of both testing the genuineness of our professed faith in Christ and then of giving assurance and comfort to the true believer.
"And hereby we do know that we know him, if we keep his commandments" (1 John 2:3).
"But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him" (1 John 2:5).
"Hereby perceive |same Greek word as 'know'| we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren" (1 John 3:16).
"My little children, let us not love in word, neither in tongue; but in deed and in truth. And hereby we know that we are of the truth, and shall assure our hearts before him" (1 John 3:18-19).
"And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us" (1 John 3:24).
"Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God" (1 John 4:2).
"We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error" (1 John 4:6).
The eighth and last such reference is our text for the day. Note that the common thread running through all is the importance of the indwelling Spirit of truth, leading those who know the Lord into lives of doctrinal purity, obedience to God's Word, and love toward the brethren. HMM