At first glance, this verse can appear troubling, or confusing at best. The key is to understand that it is describing two distinct types of people. The first are the ones who abide in Christ. Their life is immersed in Him. They regularly commune with Him and heed His voice. They live a life of continual surrender to His will. To live so closely to Christ means it is impossible to rebel against Him and sin.
Conversely, the second group consists of those who abide in sin. Just as the first group makes it their lifestyle to walk with Christ, the second group adopts sin as its lifestyle. This does not refer to those who occasionally sin, for we all do that (1 John 1:9). These people abide in sin. It is their lifestyle. It is possible to sin occasionally and to feel remorse for doing so. It is a different story for those who make it their daily routine to rebel against God. You cannot do that and be in a saving relationship with Christ.
Do you know someone who lives a life of habitual sin? Scripture declares that despite what they might claim, they do not know Christ. It is impossible to abide in the One who is infinitely holy and yet to be continually indulging in sin. Christ would not allow it. Our conscience could not bear it.