Can a believer separate themselves from Christ by sinning?
No. Nothing can separate us from the love of Christ, the love of God. A believer cannot cast himself out of the body of Christ by committing a horrible sin, because no one can touch that position any more than someone else can. The sovereign God controls believers, and nobody can supersede His power.
Les explains that he cannot see a believer separating himself from Christ. He recounts a conversation where a person argued that while God will never cast us out, a person could cast himself out by committing some horrible sin. Les responds that you cannot touch your position in the body of Christ any more than someone else can. He emphasizes that believers are totally under the control and power of the sovereign God, and nobody can supersede His power. Les says Paul is literally screaming this truth, and the closing verses of Romans 8 are like the crescendo of a great orchestra, building to the declaration that nothing can separate us from the love of Christ, the love of God. Then verse 35 goes over things like tribulation and distress.