THE ONE WHO IS SPEAKING
It seems so clear to me that the one who is speaking in Romans 7:14-25 is not only a mature Christian but an apostle at that. However, I believe the apostle is speaking in this section about himself as he is naturally (i.e. 'in the flesh' vs.14; 18) and not about his identity in Christ - the 'new man' (Eph 2:24), made alive through his 'participation in Christ' as he lives and walks in the Spirit, the faith walk described in chapters 6 and 8).
CONSISTENCY OF THE ARGUMENT
The fact is, the seventh chapter of this epistle isn't meant to produce a hiatus in the flow of the apostle's argument, as it is often made to appear, but rather his argument should slide smoothly from chapter 6 into chapter 8 as a continuous and brilliant piece of theo-logic. In other words, in chapter 7, he is following on from the point he has already made in the previous chapter (6): that we are no longer under the law but under grace.
WHY NO LONGER UNDER THE LAW? WHAT DOES THAT MEAN?
He now goes on to explain, in chapter 7 (and 8) WHY that is and WHAT that means.
WHY are we no longer under the law? Paul's answer: Because we are no longer in the flesh, "that being dead wherein we were held" (7:6; 6:15). The law speaks to the flesh and demands a response from the flesh, but, the problem is, the flesh is simply incapable of a right response, and, in fact, desires the very opposite to what the Spirit (the law derives from the Spirit) desires (Gal 5:16-17). Paul knows what his flesh is like and how it naturally reacts against the demands of the law (Rom 7:14-25) but, as he has already explained (in chapter 6), he himself no longer walks according to the flesh or lives 'in the flesh' but lives and walks in the Spirit.
PAUL'S SPIRIT WALK
He is certainly not perfect, he may still be subject to carnal moments, which is the very reason he "beats his body daily" (1 Cor 9:27), but sin is no longer the hallmark of this apostle's life (here, Paul sets the example for every Christian). However, the point Paul is making is that, if he were to still consider himself as remaining 'under that law', then he must rely on himself (i.e. on his flesh) to respond to the demands of the law, as it is to the flesh that the law speaks. Yet such a right response from the flesh, as the apostle is keen to point out, is impossible; hence, in order to drive his point home, his description of how his flesh ('the natural man') still functions (even after conversion - it is the mind that is converted, not the flesh),though he himself no longer 'walks in in the flesh': (7:15-21).
SAME ARGUMENT PRESENTED TO THE GALATIAN CHRISTIANS
As Paul explained to believers in Galatia, if we, having been delivered from bondage, continue to 'walk in the flesh' then "[we] cannot do the things that [we] would "(Gal 5:17; Rom 7:15).
PAUL'S SUMMARY OF THE ARGUMENT SO FAR
Paul sums up what he has said so far, like this (v.25): "So then [if I remain 'under the law' and therefore try to respond 'in the flesh' to the law's demands] it is true that, with my mind I [may] serve the law of God (that's where my desire is) but with my flesh, [I can only serve] the law of sin [the flesh will always pull me in the opposite direction to where the Spirit wants me to go, if I choose to rely on or continue to 'walk in the flesh'. The natural man simply has no choice but to sin, given the sinful nature or 'law of sin' within v.23]." Condemnation and death will be the result (v.11).
NO LONGER IN THE FLESH
It now becomes clear why Paul regards himself as no longer 'in the flesh' (ruled by the flesh) but 'in the Spirit' (ruled by the Spirit) and why he is no longer 'under the law', because the law speaks to the flesh, not to the Spirit (Gal 5:23), and demands a flesh response (1 Tim 1:9).
NO LONGER UNDER THE LAW, BUT UNDER GRACE
WHAT does it mean that we are no longer under the law (i.e. no longer 'in the flesh')? The answer is in the second part of Romans 7:25 (the cry in the first part is rhetorical) "I thank God THROUGH Jesus Christ our Lord". It is Paul's (and every Christian's) 'participation in Christ' which enables him or her to escape the flesh and live and walk in the Spirit. It is Christ's life in us, not our own natural life which is the source of the new creation (the 'new man' Eph 2:15; 4:25; Col 3:10). It is through Christ's indwelling, that we are able to "serve in newness of spirit, and no longer in the oldness of the letter (Rom 7:6)."
THANK GOD FOR WHAT?
But thank God for what? Does the apostle tell us? Yes, he does so in the previous chapter: "But God be thanked, that [I was once] the [servant] of sin, but have [now] obeyed from the heart (i.e. have attained to the righteousness which is by faith alone in Christ alone)" (6:17).
WHAT IT MEANS TO WALK IN THE SPIRIT
Walking in the spirit is a faith walk and a 'participation' in the life of the risen Lord; by which our 'old man' is crucified with Christ (Gal 2:20) and we walk in newness of life (Rom 6:4).
And what does this all mean? It means "...therefore [since we are no longer in the flesh and no longer under the law, having explained why we can no longer continue in the flesh]. there is now no condemnation for those who are in Christ Jesus who walk not after the flesh but after the Spirit (Rom 8:1).