Notice that the verse says, "if we say". If we say that about ourselves then our sin remains; that is, if we believe that we have removed sin from ourselves, sin remains. But, we do not say that. Instead, it is the Bible that says that about us - that through Christ, we have no sin.
[1Jo 1:8 KJV] 8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. The Bible tells us so in the following verse:
[1Jo 3:9 KJV] 9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
We are under grace, not law. Where there is no law, sin cannot exist because it cannot be assessed: without law, there is no standard for the measurement or assessment of sin:
[Rom 7:8 KJV] 8 But sin, taking occasion by the commandment, wrought in me all manner of concupiscence. For without the law sin [was] dead.
Have to stop for now - wife needs laptop. Will finish when I can - sorry.