1John 1:10

If we say that we have not sinned, we make him a liar, and his word is not in us.
(1 John 1:10 KJV)

"If we say that we have not sinned..."
If this is not the 'sin' we discussed in the commentaries for 1Jn 1:7-8 and 1Jn 1:9, it must be another possible two instances:

CASE I

If we say we have not sinned when we have
and can evidently see what wrong we did but we decide to put up an unrepentant attitude, oh no! we're playing on the path that yields us no profit. For he says "I will instruct thee and teach thee in the way which thou shalt go: I will guide thee with mine eye." (Psalms 32:8) "Now if you are exempt from correction and left without discipline in which all [of God's children] share, then you are illegitimate offspring and not true sons [at all]. (Hebrews 12:8 AMP)  

CASE II

If we say that we have not sinned... as in our pre-salvation life. Why did I conclude so? We must ask ourselves when/where did "Him" say we sinned, such that our saying otherwise is to make him a liar?

God looked down from heaven upon the children of men, to see if there were [any] that did understand, that did seek God. Every one of them is gone back: they are altogether become filthy; [there is] none that doeth good, no, not one. (Psalms 53:2-3 KJV). (See also Genesis 6:5 and Jer 17:9).

However it became an entirely new story with  the advent of the Second Man - Jesus Christ, "for he shall save his people from their sins." was the prophecy! (Matt 1:21). Thank God it has been fulfilled and it's still being fulfilled today.

So to deny that we never sinned before we encountered Christ is to "lie" and know not the truth.

Thank God for whom we are now that we have received Jesus. He has sanctified us and given us the right of sonship (John 1:12)
Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ:  (1Pet 1:2) 

1John 1:9

Please read our commentary on the previous verse I John1:7-8

(1 John 1:9 KJV)

If we confess our sins, he is faithful and just to forgive us [our] sins, and to cleanse us from all unrighteousness.

If you have understood what 1John 1:7-8 says as explained, then you would not have problem with this verse.
We must re-emphasize that the Writer never meant to say  that we Christians who are called the 'saints' by the Holy Ghost in the epistles are 'sinners' or that we are still in sins no matter how we try. How do I know this?

1John 1:7-8

1Jn 1: 7-10

(1 John 1:7 KJV)
But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.

(1 John 1:8 KJV)
If we say that we have no sin, we deceive ourselves, and the truth is not in us.

(1 John 1:9 KJV)
If we confess our sins, he is faithful and just to forgive us [our] sins, and to cleanse us from all unrighteousness.

(1 John 1:10 KJV)
If we say that we have not sinned, we make him a liar, and his word is not in us.

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We bless God for the reading of His Holy word!
What actually is the Apostle John talking of as he through the Holy Spirit writes to the church of Christ?  What is this concept on sin he is talking about? The Blessed Lord open our eyes of understanding to it in Jesus name.