The understandable concern, and I agree with the concern, is that we need to not use this passage as an excuse to say, "Oh, I'm just struggling with obedience, but I'm really a believer." Our lawless/antinomian impulse is always strong, but so is our legalistic impulse, and it seems that we all seem to be able to use certain passages to buttress our preconceived notions of what we think Scripture ought to say about the "normal" Christian life. I'm inclined towards this passage because I do struggle with sin daily. Thus it gives me comfort. But I also admit that I can easily rely on this passage to excuse my sin. But though this is clearly the case, even if I'm guilty of this sin, it doesn't necessarily negate the truth of that view of this passage. The "struggling believer" interpretation of Romans seven may well be the right interpretation, even if it is misused. Likewise, just because I abhor legalism, doesn't mean that the other option is not possible either. It may well be true that it is describing the unregenerate. It may as well be true that to be Christian is to be transformed in such a way that this passage cannot be an accurate description of the daily Christian walk. I'm certainly open to the arguments on either side. My personal weaknesses should not determine how I read the text. If I do that, I end up standing in judgment over Scripture, instead of the other way around. Scripture itself declares in no uncertain terms that it stands in judgment upon us (Hebrews 4:12, 2 Timothy 3:14-15), and can make us wise unto salvation.
So, in light of this reality, we must do the leg work of balancing out the various passages about who we are in Christ. Yes, we are new creations. Yes, our old man/woman has been declared dead. Yet we are also told to continually put to death our old man. So, while he is "declared" dead, he still kicks around so long as we live in our current body. Like the kingdom of God itself, which is an already/not yet reality, we seem to also inhabit an already/not yet state in our being new in Christ. Maybe a visual/graphic expression may be of some help in better understanding this:
It is in this intervening period of life between our conversion and our final resurrection that we inhabit this duality of the old and new both existing in us, though with the old dying away (being put to death) and the new growing into fullness (putting "on" the new man).
Our pastor is understandably concerned to bring out that we have at our disposal so much more than what we realize in Christ through His Spirit indwelling us. CS Lewis made the point well when he said that our problem is not that we ask for too much, but that we are far too easily satisfied and ask for far too little.
While I still believe that Romans seven is talking about the believing Paul (and thus us in Christ as well), it is not describing the life of a "defeated" Christian. It may well be describing the transitional/sporadic period of what a believer experiences upon trying to measure up to God's perfect standard in their own strength, apart from His power through the Spirit of Christ. Paul, in his heightened conscience, may well be describing what Isaiah described in Isaiah six when he was confronted with the awful holiness of God. This isn't a description of an unbeliever, or even of a defeated believer, but is the natural expression of a moment of realization of God's utter holiness and righteousness. It provokes awe and fear and self loathing, yet with the end result of being reconciled with this same God, thus ending in inexpressible joy. And in fact, that is exactly how Paul ends that section of Romans. Thanking God through Jesus Christ our Lord! This both gives hope to every believer and warns against a cavalier attitude about Who God is and what He requires. Were it not for grace!