The person who has been finally "born of God" will not sin any longer. Once we are born of God at the resurrection, we will be able to live without ever sinning again. How? Simply because we will then possess the fullness of God's perfect character and divine nature. We will no longer possess a nature which can fall victim to sin and Satan's influence.
But as long as we are only "begotten" of God, we must "keep ourselves"—we must continue to resist the evil temptations of the flesh. Such a one can commit sin. When caught off guard or in a moment of weakness, we can and often do sin! But we cannot "practice [or, continually live in] sin" and be the begotten sons of God (I John 3:5-6).
Life, to the begotten Christian, is a constant struggle against Satan's influence, which is the cause of the evil side of man's nature. Although he has now received the begettal of God's divine nature (II Peter 1:4), he still manifests the traits of human nature as well, and the one wars against the other (Galatians 5:17).