W
Well a child of God is justified by Christ, and in Him we cannot sin. I hope you see the difference between saying you don't sin and saying in Him you cannot sin?
The flesh will always be an obstacle to perfect submission in this life. Our flesh will always have thoughts which war against the Spirit, but the more mature we become the less and less they occur and the less and less they have any power over us whatsoever. We just get used to taking them captive immediately and dismissing them. Not giving them any place in our inner man for our desires become loving. We begin to despise those fleshly thoughts and they become more of an annoyance than anything. Like a little fly that buzzes around our head from time to time and is easily swatted away.
But in order to get to that point we have to pick up our cross and start following.