No one who abides in him keeps on sinning; no one who keeps on sinning has either seen him or known him (1 John 3:6).
This is a sobering passage. It is meant to be. However, it should not be a passage that causes the Christian believer to despair. John is not condemning the one who is seeking to overcome sin, but the one who habitually gives into sin without repenting. John purposefully describes here the one who is (or is not) continually sinning (as is evident from the “linear present”, or continuing action, of the verbs in the original language).
We also know this from some other passages which have gone before in this same epistle: “If we say we have not sinned, we make him a liar, and his word is not in us” (1 John 1:10) and “My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous” (1 John 2:1).
How can we be certain of our own salvation? By continually coming to God, through Christ, in repentance and faith.
The well-rounded truth John is teaching, then, includes multiple simultaneous realities: 1) that our goal should be not to sin at all; 2) that we will commit some sins nonetheless; 3) that as long as we are continuing in a state of rebellion and sin we have no biblical assurance of our salvation; and 4) that, when we do sin, we have an advocate with the Father, Jesus Christ the righteous.
It seems God has purposely withheld assurance from those who are walking in constant sin and rebellion. And, yet, God also wants those who come to him in repentant faith to know that they will be seen in the righteousness of Christ, not in their own imperfect righteousness. Therefore, each time that we willfully sin, we rob ourselves of the comfort and assurance of our salvation. And, yet, each time we turn again in repentance, God assures us of a ready and open-armed reception, because of the righteousness of Christ.
How can we be certain of our own salvation? By continually coming to God, through Christ, in repentance and faith.