6No one who lives in him keeps on sinning. No one who continues to sin has either seen him or known him.

7Dear children, do not let anyone lead you astray. He who does what is right is righteous, just as he is righteous. 8He who does what is sinful is of the devil, because the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the devil's work. 9No one who is born of God will continue to sin, because God's seed remains in him; he cannot go on sinning, because he has been born of God. 10This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not a child of God; nor is anyone who does not love his brother.

1 John 3: 6-10

1 John 1:8: “If we say we have no sin, we deceive ourselves,
and the truth is not in us.” We! We born-again Christians.
In other words, don’t let the deception of these false teachers
work its way into your own self-deception. There are no sinless
Christians

1 John 2:1: “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.” In
other words, John does not assume that if you sin, you are not
born again. He assumes that if you sin, you have an Advocate.
And only those who are born again have this Advocate.

1 John 5:16-17: “If anyone sees his brother committing
a sin not leading to death, he shall ask, and God will
give him life—to those who commit sins that do not lead to
death. There is sin that leads to death; I do not say that one
should pray for that. All wrongdoing is sin, but there is sin
that does not lead to death.”
Notice that last clause: “There is sin that does not lead to death.”
This is why you can see your brother committing sin. He is your
brother. He is born again. And he is sinning. How can this be?
Because there is sin that does not lead to death. I don’t think
John has particular kinds of sins in view, but rather degrees of
rootedness and habitual persistence. There is a point of confirmed
sinning which may take you over the line of no return and you
will be like Esau who sought repentance with tears and could
not find it (Heb. 12:16–17). He could not repent. If he could have,
there would have been forgiveness. But the heart can become so
hardened by sin that even its desires to repent are counterfeit

Adapted from "Finally Alive" by John Piper