Problem: John affirms here that “Whoever has been born of God does not sin.” But in the first chapter he insisted that “If we say that we have no sin, we deceive ourselves, and the truth is not in us” (1:8).
Solution: John nowhere claims that believers are without sin or never commit a sin. First John 3:9 is in the present continuous tense and should be translated “Whoever is born of God does not continually practice sin.” Conversely, if a person habitually practices sin, he is not born of God. As James argued, true faith will produce good works (James 2:14ff). If a pig and a lamb fall into the mud, the pig wants to stay there, but the lamb wants to get out. Both a believer and an unbeliever can fall into the same sin, but a believer cannot stay in it and feel comfortable.
 Geisler, N. L., & Howe, T. A. (1992). When critics ask : a popular handbook on Bible difficulties (p. 539). Wheaton, Ill.: Victor Books.