Most protestant denominations I'm familiar with would deny believers never sin after they've received eternal life and initial saving faith in Christ. Rather, while they remain in fellowship with God, they are dead to sin or separated from the penalty of sin. We become free from the bondage of sin because we have already the first hope satisfied in us, namely eternal life. Our next hope is in our remaining in fellowship with Him, putting on the mind of Christ, recognizing there are rewards for us in heaven based upon our good works performed through faith in Him when we perform according to His will and Plan. We also recognize we stumble repetitively in sin after salvation prior to the first death, but by calling upon His promises of 1stJohn 1:9 we still have life through faith in Him. Even though we are slaves to righteousness, we might still sin from the influence of the old man, disobey in our volition, but we nevertheless remain His and slaves to Him forever.
On this very thread (I don't want to mention any names) a Reformed Protestant Freeper basically said that if one sins he is not a Christian. So, in part I was reacting to that statement, which seemed rather incredulous.
Rather, while they remain in fellowship with God, they are dead to sin or separated from the penalty of sin
I am not sure I understand the terminology here, or the concept for that matter. But I think this is different than what +Paul is saying in Rom 6. He says rather definitively that those who have been Baptized (accepted Christ as their Savior) are dead to sin and sin no more.
I have also been told by Protestants on this Forum that (1) the elect cannot fall away because (2) God won't let them. From the way you are stating it, I get the impression that we determine if we are going to stay "in fellowship" with God and not the other way around.
Trouble is, we are not dealing with a 'Protestant doctrine' but with individual Protestants who believe, let's be brutally honest, whatever they want to believe, or whatever they think the indwelling Spirit leads the to believe.
As such, a coherent general discussion regarding doctrine or theology is severely limited, as there will always be another Protestant who is likely to disagree with the first one.