I John 3:4 “Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.”
Under the umbrella of the truth that “every man that hath this hope in him purifieth himself, even as he is pure” (I John 3:3), John now defines “sin” as “the transgression of the law”. Thus, to transgress “the law” is to be impure. The tense of the word “purifieth” emphasizes that, for man, this purifying is a perpetual struggle; for, while the Spirit abides within us, we are still plagued with a human nature which lusts to “sin”; thus, we are prone to “sin”. Man’s perpetual struggle against “sin” is contrasted to the truth that our Blessed Lord “hath” eternally “saved us, and called us with an holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began” (II Timothy 1:9). That is, our struggles are perpetual; but, by the singular act of His Sacrifice, Our Lord completely and finally saved “his people from their sins” (Matthew 1:21); thus, He plainly declared, “It is finished” (John 19:30). But, being prone to “sin”, those who sincerely love the Lord, loath their “sin” and diligently strive to show their love to our Lord by keeping His Commandments (John 14:15); that is, not transgressing “the law”. The Old Testament Sacrificial Laws and the myriad of Laws for ceremonial feast and such were fully kept and satisfied by our Lord, Jesus Christ (Matthew 5:18). But, both the Old and New Testament Laws for faithful obedience to our Lord, meeting to worship, and keeping His Commandments for morality, integrity, brotherly love, etc. are our charge as long as we live in this present world. To transgress these laws is to “sin”, and persistence in “sin” leads to shame and pain and ruin and ultimately, death. James explains it this way: “But every man is tempted, when he is drawn away of his own lust, and enticed. 15 Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death. 16 Do not err, my beloved brethren.” (James 1:14-16). It is a painful truth that “all have sinned, and come short of the glory of God” (Romans 3:23). But, those who “purifieth” themselves, often hear our Lord lovingly speaking in their spiritual ears, “Well done, thou good and faithful servant” (Matthew 25:21)!