Paul says it is not enough to hear
God's law, it has to be obeyed
Why do some Christians treat God's law with such contempt? These brothers proceed as if they thought the law was invented by Satan. In this verse Paul himself, who some people consider the champion of the law haters, assures the contrary to what they think. He says it was not just the hearers of the law, but the doers of the law who act correctly. The law that Paul considers obsolete is the ceremonial laws, not the behavioral laws.
For not the hearers of the law are just before God, but the doers of the law shall be justified. (Ro 2:13)
Paul is saying here that we have to obey God's law for human behavior, that we must align our behavior by that is established in God's law.
Does that mean that salvation comes by finalizing a life after having lived obeying the law without fail? No, no one can accomplish that; that is why the Lord came. Only Jesus did it. And we are saved thanks to what he did and his sacrifice to accomplish what we could not do ourselves. But nobody is saved either by saying he believes in Jesus, he lives under the grace, and then goes on to consciously disobey God's law, voluntarily and premeditatedly.
The Christian who sincerely believes that what was established by God does not have to be obeyed is going to suffer the local and temporary consequences of not obeying God's law, but doesn't stop being saved, because his sin is unconscious. If he knew he had to obey he would do it, and repent from not having obeyed before.
The Christian who does not obey what God has established because of circumstances, pressures, weaknesses or temptation, even when he was conscious he had to obey, suffers the local and temporary consequences of his sin. But if he sincerely repents of his weakness, or fights the temptation that led him to error, without giving in to it, repentant for having let himself be deceived, does not lose his salvation either, because he did not do it voluntarily, but under human or demonic pressure. A good example of this would be Peter's denial.
A Christian, whom, surprised by sin, falls in it, will no avoid the local and temporary consequences of his sin, but if he sincerely repents he does not lose his salvation because his sin was not premeditated.
All this is true; but from there to affirming that what was established by God does not have to be obeyed, because we are under grace and we can disobey to our heart's delight, there is a large and deep abyss; and that abyss has a name: let's try not to fall in it.