When believers trust in Yeshua for their salvation, they become new creatures. The old carnal nature is dead and they are freed from sin and are able to become servants of righteousness. Paul continued with this:
Romans 7:1-6 (KJV)
Know ye not, brethren, (for I speak to them that know the law,) how that the law hath dominion over a man as long as he liveth?
For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband.
So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man.
Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God.
For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death.
But now we are delivered from the law, that being dead wherein we were held; that we should serve in newness of spirit, and not in the oldness of the letter.
Although Paul was continuing his discussion from chapter 6, he expanded his thought by drawing on another analogy, this time from the laws regarding marriage. He began by asking his readers another leading question (verse 1). Don't the believers know that the Torah has dominion over a man as long as he is living? He added a parenthetical comment to make his readers aware that the coming argument was written with those in mind who understood the Torah. This undoubtedly was a reference primarily to Jews, although because the Roman congregation was largely Gentile, there seems to be an understanding on Paul's part that many of the Gentiles were at differing stages of understanding Torah.
In verse 2 Paul explained Torah law regarding marriage. A married woman is bound by Torah to her husband as long as he lives. But if the husband dies, she is free from that marriage.
Further, if while the husband is living, the woman marries another man, she is an adultress. However, if her husband is dead and she marries another man, she is not an adultress (verse 3).
Before going on to verse 4, it must be understood that in Paul's analogy, the woman, upon her husband's death, is no longer subject to the Torah law that says she cannot marry anyone else. The Torah itself and all the laws regarding marriage are still in force. She simply has the legal ability to marry someone else.
In the same way, believers become dead to Torah by the death and resurrection of Messiah (verse 4). This is not saying that Torah or any laws of Torah are done away with. It simply means that the believers, because of trusting in Yeshua's death and resurrection for salvation, are no longer subject to the condemnation of Torah and the penalties sin requires. The believers are, in essence, able to "marry" or serve someone else besides the fleshly nature. That someone is Yeshua. The believers are then able to bring forth fruit unto God.
For prior to this, the believers were in the flesh (they lived according to their fleshly, carnal natures) (verse 5). The sin they committed which were by the Torah (the Torah showed what sin was and then brought condemnation on those who sinned) , worked in their body parts to bring forth fruit unto death.
But believers, upon salvation, are delivered from the condemnation of Torah, their old natures are dead, and they are able to serve in newness of spirit, rather than in the oldness of the letter. The Torah still exists, still points to sin, but believers have new natures (hearts) upon which the Torah is written. The believers receive power and desire to live Torah, rather than living under the Torah's condemnation.
In summary: believers are originally "married" to their fleshly natures (the old man), but after the fleshly nature is dead, they are free to "marry" Yeshua and serve Him in righteousness. Hallelu-Yah!