By no means! We are those who have died to sin; how can we live in it any longer? (Romans 6:2)

For we know that our old self was crucified with him so that the body ruled by sin might be done away with, that we should no longer be slaves to sin— because anyone who has died has been set free from sin (Romans 6:6-7).

But thanks be to God that, though you used to be slaves to sin, you have come to obey from your heart the pattern of teaching that has now claimed your allegiance. You have been set free from sin and have become slaves to righteousness (Romans 6:17-18).

All of these verses seem to suggest that once you are saved in Christ you can no longer sin. Is this what Paul is saying? Yes and no. 

If we have died to sin, it is true that we cannot sin any longer. But the fact is that we have not died to sin because the sin nature still lives within us. That is why Paul also writes, 

For the sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want (Galatians 5:17).

It is also why Paul writes,

Therefore, brothers, we have an obligation—but it is not to the sinful nature, to live according to it. For if you live according to the sinful nature, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live (Romans 8:12-13).

And if our old nature was crucified, it is true that we would be set free from sin. But the fact is that our old self has not yet been crucified. This is why Paul admonishes believers,

You were taught, with regard to your former way of life, to put off your old self, which is being corrupted by its deceitful desires; to be made new in the attitude of your minds; and to put on the new self, created to be like God in true righteousness and holiness (Ephesians 2:22-24).

And it is true that if we are no longer slaves to sin, we would obey God from the heart. But the fact is that we are still in part slaves to sin. That is why Paul writes at the end of Romans 7,

So then, I myself in my mind am a slave to God’s law, but in the sinful nature a slave to the law of sin (Romans 7:25).

Since none of the realities Paul speaks of is true yet, where does he get off making such arguments and assertions?! This is the real question.

To Paul, being justified is to be glorified. Even though, as believers, glorification is still future, Paul speaks of it as past because it is so certain: 

And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified (Romans 8:30).

And even though we have not died to sin yet, Paul speaks of it as a done deal now. Even though our old nature will be crucified in the future, he speaks of it as a realized fact. Even though it will not be till heaven when we become free from slavery to sin, Paul speaks of it as a declared fact. Why? 

It is becasue it is certain. What God began, He will complete! (Philippians 1:6). Paul is convinced that God will preserve every Christian until glory:

Yet I am not ashamed, because I know whom I have believed, and am convinced that he is able to guard what I have entrusted to him for that day (2 Timothy 1:12).

So, what is Paul's intention in writing things still future as a present reality? I believe he is trying to encourage believers to live today in light of the certain glory that awaits them. That is why he writes, 

In the same way, count yourselves dead to sin but alive to God in Christ Jesus (Romans 6:11).

Paul wants believers to consider it so, to reckon it so, to think it so. In another place he writes,

Since, then, you have been raised with Christ, set your hearts on things above, where Christ is, seated at the right hand of God. Set your minds on things above, not on earthly things (Colossians 3:1-2).

Our example is Christ, of whom we read,

For the joy set before him he endured the cross, scorning its shame, and sat down at the right hand of the throne of God. Consider him who endured such opposition from sinners, so that you will not grow weary and lose heart (Hebrews 12:2).

Therefore, it will be true that those that in Christ can no longer sin. That reality will come to full realization in heaven. But our present reality is one of struggle, where we daily fight against our sinful natures and put to death the misdeeds of our bodies. We ought to live now, therefore, with the certain hope of our future glory. Our conversation, our attitude, and our actions should not be shaped by some hypothetical notion of future glory but the absolute certainty of it.