Why do believers still sin if they have a new nature?
Believers still sin because the old Adamic sin nature, though crucified in God's reckoning when they believed the Gospel, remains experientially active; the new divine nature cannot sin, but when a believer sins, the old nature has temporarily taken control.
Les explains this from Galatians chapter 5, verse 17, using the two natures of the believer. When a person becomes a believer, the old sin nature—the old Adam we are born with—had to be crucified and put to death in the realm of the Spirit, which God accomplishes when we believe the Gospel. Then God immediately places opposite that old Adamic nature a new nature, a new divine nature, and Les maintains that this new nature cannot sin because it is divine. So when believers sin, it is a simple matter that old Adam has taken control for a moment or so, and it is through the old nature that we continue to sin. Les emphasizes that in God's eyes old Adam is dead, which is why He can declare us justified, just as if we had never sinned; but experientially, in our everyday life, old Adam is still the source of the believer's sin.
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
More Questions
What happens to the old sin nature when a person becomes a believer?
When a person becomes a believer, their old sin nature (the old Adam) had to be crucified and put to death in the realm of the Spirit, which is done when they believe the Gospel and Christ reckons it as crucified.