"If we claim we have no sin, we are only fooling ourselves and not living in the truth. But if we confess our sins to him, he is faithful and just to forgive us our sins and to cleanse us from all wickedness. If we claim we have not sinned, we are calling God a liar and showing that his word has no place in our hearts." (1 John 1:8-10)
"Those who have been born into God’s family do not make a PRACTICE of sinning, because God’s life is in them. So they CAN'T KEEP ON sinning, because they are children of God."
(1 John 3:9)
If you read the verse "if we claim we have no sin", the word sin is singular. It does not refer to sins (plural) in action but points out to the nature of sin itself. So if we claim we have no nature of sin, if we say we are perfectly pure and we cannot commit sins any more, we are liars because God did not say that. If we don't have that sin nature in us we would not need a lawyer, an advocate, to represent us and plead for our case (1 John 1:7 and 1 John 2:1); neither do we need the blood of Jesus Christ to apply by confession during our Christian life.
The verse that says, the "blood of Jesus Christ will cleanse you from all sins" belongs to us, believers, during our lives on earth. Yes, any believer who does not say or does not confess sins during his/her life will not be covered under the blood of Jesus Christ. It is interesting that many believers think that all their sins in the past, present, and future are already cleansed; but in reality it is not so true. All the sins in the past were under Adam family (I call it the "first book") and the sins in this book were all washed by the blood of the cross. But now that we are transfered to God's family, we have now a "second book". This second book is open (the book of Life). But to keep a healthy relationship and fellowship (communion), we must exercise 1 John1:7-- confession. If we don't, our Christian lives will not produce fruits to please the Gardener and rewards will not be there for us. The judgment seat of Christ in 2 Corinthians 5:10 will also demonstrate our inner character--good or bad. Rewards are in view here. The Lord did not only save us for ever but he gave us a chance to get extra bonuses. We can get rewards with the fact that we are already saved.
Yes, saved once saved for ever. But we have obligation to confess our sins to keep the communion between heaven and earth for a better relationship between us and Them (the Trinity). We can do whatever HE wants!