"Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. And ye know that He was manifested to take away our sins; and in Him is no sin. Whosoever abideth in Him sinneth not: whosoever sinneth hath not seen Him, neither known Him. Little children, let no man deceive you: he that doeth righteousness is righteous, even as He is righteous. He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil. Whosoever is born of God doth not commit sin; for His seed remaineth in him: and he cannot sin, because he is born of God." (1 Jn. 3:4-9 KJV)
Because we know born again believers still sin (see Rom. 7:14-25; Jam. 5:13-16; 1 Jn. 1:8-10), I like how the New Living Translation has this passage:
"Those who sin are opposed to the law of God, for all sin opposes the law of God. And you know that Jesus came to take away our sins, for there is no sin in Him. So if we continue to live in Him, we won't sin either. But those who keep on sinning have never known Him or understood who He is. Dear children, don't let anyone deceive you about this: When people do what is right, it is because they are righteous, even as Christ is righteous. But when people keep on sinning, it shows they belong to the Devil, who has been sinning since the beginning. But the Son of God came to destroy these works of the Devil. Those who have been born into God's family do not sin, because God's life is in them. So they can't keep on sinning, because they have been born of God." (1 Jn. 3:4-9 NLT)
There are several reasons a born again believer cannot continue to practice sin. For one, he is a child of God, and God will not tolerate it (Heb. 12:5-11). For another, the Holy Spirit within brings conviction to believers (Jn. 14:26; 16:13) and grieving the Holy Spirit could even bring death (Acts 5:3-4; 1 Cor. 11:28-30; Eph. 4:30). Still another is that when we continue to sin, we do not lose our salvation, but we do lose the joy of our salvation (Gal. 5:22-23; 1 Thes. 1:6). Failing to walk in the Spirit (abide in Him throughout our day), makes a born again believer miserable (Gal. 5:16-25).
Judge yourselves: are you "practicing righteousness," or "practicing sin?"