Romans 2:12-15 talks about the Gentiles being a law unto themselves. What does this mean?
Romans 2:12-15 in some different translations:
12 For all who have sinned without the Law will also perish without the Law, and all who have sinned under the Law will be judged by the Law; 13 for it is not the hearers of the Law who are righteous before God, but the doers of the Law who will be justified. 14 For when Gentiles who do not have the Law instinctively perform the requirements of the Law, these, though not having the Law, are a law to themselves, 15 in that they show the work of the Law written in their hearts, their conscience testifying and their thoughts alternately accusing or else defending them,
12 For as many as have sinned without law will also perish without law, and as many as have sinned in the law will be judged by the law 13 (for not the hearers of the law are just in the sight of God, but the doers of the law will be justified; 14 for when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, 15 who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them)
To understand any passage properly, we need to be aware of the context in which it is said. The overall context of Romans is dealing with difficulties between Jewish and Gentile Christians working together. The Jewish Christians often seemed to think they were superior to the Gentile Christians and Paul uses the first chapters of Romans to destroy this idea. He then destroys the idea that Gentile Christians are superior to the Jews. We are all equal in the sight of God and all need the same gospel. We must find a way to work together.
At this point, Paul has just talked about the sins of the Gentiles in chapter 1 and then turned to the problems the Jews faced in chapter 2. But here is what he says about the Gentiles at the end of chapter 1. Romans 1:28-32 (NASB) reads:
28 And just as they did not see fit to acknowledge God, God gave them up to a depraved mind, to do those things that are not proper, 29 people having been filled with all unrighteousness, wickedness, greed, and evil; full of envy, murder, strife, deceit, and malice; they are gossips, 30 slanderers, haters of God, insolent, arrogant, boastful, inventors of evil, disobedient to parents, 31 without understanding, untrustworthy, unfeeling, and unmerciful; 32 and although they know the ordinance of God, that those who practice such things are worthy of death, they not only do the same, but also approve of those who practice them.
He says here that they know the ordinance of God. These are not people who have absolutely no knowledge of God but rather those who do know something and yet do not see fit to acknowledge God. They did not have the law of Moses but they were not without any law at all.
We then find that he turns to the Jews in chapter 2 and is showing that while they condemn the Gentiles for all of these sins, they themselves are just as guilty. He makes the point that there is no partiality with God (verse 11). He then enters into the verses we are now looking at.
Sin with or without the law
The idea of those who sinned without the law is that Gentiles never had the Law of Moses and so they would not be judged by it. However, they still had law. That is why he says they will perish without the law. But those who had the law (the Jews) were not really any better off regarding sin because it is not the hearers of the law that are justified but the doers of the law. Since they had the law but broke the law, they would also perish as they were judged by the law. Everyone needs the gospel just as much as everyone else.
Instinctively Perform the Requirements of the Law (By Nature Do the Things Required By the Law)
Here, Paul cannot be referring to the laws that were particular to the Jews. The laws regarding worship at the Temple, the priesthood, clean and unclean animals, etc. were not something required of Gentiles. Actually, some of those laws had specific exclusions for the Gentiles how lived among the Jews. For example Deuteronomy 14:21 allows Gentiles who lived among the Jews to eat meat from an animal that died by itself but the Jew was not allowed to eat it.
The requirements of the law that the Gentiles instinctively (or by nature) knew to do are the same kind of laws he talked about in chapter 1. In chapter 1, he was focused on them breaking those laws but he makes it clear that these are things they could know about God and did know (Romans 1:32). He does not say in particular how the Gentiles come to know these things other than what he says in Romans 1:18-20, which reads (NASB):
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of people who suppress the truth in unrighteousness, 19 because that which is known about God is evident within them; for God made it evident to them. 20 For since the creation of the world His invisible attributes, that is, His eternal power and divine nature, have been clearly perceived, being understood by what has been made, so that they are without excuse.
There are certain things about God that can be understood simply through nature. When we understand those things about God, we can understand some things that He would want from us. If He created all of us, how can we think that He wants us to commit murder or steal from each other or lie to each other? There are things that some people in all cultures have understood to be right and wrong whether they ever had seen the Law of Moses (or even the Law of Christ) or not.
Their Conscience Testifying
When we become convinced that something is right or wrong, our conscience is there to accuse us when we do what we believe is wrong and excuse use when we do what we believe is right. This does not mean that as long as someone follows their conscience they are alright with God. This does mean that when their conscience is trained in truth (by nature doing the things of the law) their conscience is a powerful aid in doing what is right and is essentially a subsitute for what the Jews had with the Law that accused or excused them.
What Paul is addressing here is the idea among the Jewish Christians that the Gentiles Christians came from a background entirely missing any moral compass at all. They looked down on their Gentile brethren because of this. Paul is explaining that many of the Gentiles were better than many of the Jews and how that could be true since they did not have the Law. In the end, it makes no difference because all have sinned and come short of the glory of God. All need the same gospel to be saved. We cannot look down on our brethren because we have been just as much sinners as they have been.