In what sense was there ever no law? When did God's moral law ever lower? When was God's law never written on the hearts of men?
"First, let me admit that there are scholars who understand the law written on the heart in Romans 2:15 and the law written on the heart in these new covenant passages like Hebrews 10:14 as referring to the same experience of transformation by the Holy Spirit in the heart of the Christian. That view is more common today. Very few took that view historically. It is not my view. So, let me read those texts, say what my view is, try to explain what the implications are, and hopefully be of some help...
Let's start with Romans 2:14: "For when Gentiles, who do not have the law, by nature" and I emphasize that phrase because it ties this passage in to Romans 1 where Paul was not dealing with redeemed people and what they do by virtue of the Holy Spirit in their heart, but things that they do by virtue of being created in a human nature or a male and female nature.
So, let's start over: "For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law" meaning the law of Moses. "They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them" (Romans 2:15).
"Every human being in the world has an inborn knowledge of God and his law." Tweet Share on Facebook
So, the picture here, I think, is not of a born again, redeemed people who are enabled to walk in a way pleasing to the Lord by the Holy Spirit, but of the Gentile world in general who have enough knowledge of the moral law of God in their hearts by virtue of being created in God's image so that their consciences are conflicted: sometimes approving, sometimes disapproving.
The other passage is Hebrews 10:1617. He is quoting the new covenant promise in Jeremiah 31: "'This is the covenant that I will make with them after those days, declares the Lord: I will put my laws on their hearts, and write them on their minds,' then he adds, 'I will remember their sins and their lawless deeds no more.'" Now, the point here is that Christ has purchased the new covenant promise, and it includes the forgiveness of our sins and the replacement of an old, unbelieving, rebellious heart with a new heart of faith and obedience. So, I think these two passages Romans 2, Hebrews 10, and the other new covenant passages like 2 Corinthians 3 and Hebrews 8 are teaching two very different things and addressing two very different situations.
Hebrews is teaching that, when we are born again, God gives us a new heart and a new spirit, and the result is that the law of God written in Scripture is no longer offensive to us. We are no longer hostile to it like Paul says in Romans 8:7. We are not hostile to the word of God, but rather, we are submissive. We delight to do God's commandments. It doesn't mean that we know them all by heart, because they are written on our hearts. It doesn't mean that. It means that, when we read them in the word, the inclination to do them is in our hearts by the Holy Spirit. This is a great work of the Holy Spirit, purchased by the blood of Christ called the new covenant. That is what I think is going on in Hebrews 10 and it is an awesome privilege for Christians.
In Romans 2, what Paul is doing is very different. Let's get the train of thought from verse 11 down to where we are jumping in at verse 15. Let's get that in front of us. First he says and this is the main thesis of the paragraph there is "no partiality" with God. And then he defends that in verse 12 by saying that God's judgment will fall according to how we respond to the measure of truth that we have access to. That is why he is not partial. Then he explains this in verse 13, that hearing the law of God is no advantage to the Jew. In other words, merely having it and hearing it is no advantage to the Jew at the judgment day, and not hearing it is no disadvantage to the Gentile, because doing, not hearing, is the issue. And then he explains in verses 14 and 15 that the law really is available to those who have no copy of the Mosaic law, because God has put it in the hearts and given all of us a conscience to awaken us to this moral knowledge in our hearts.
What he means is not that every Gentile unbeliever in the world knows all six hundred commands in the first five books of the Bible. What he means is that all human beings have sufficient knowledge of what is right and wrong written on their hearts, so that their consciences can accuse them or affirm them. And at the judgment day they will not be held accountable for what they have no knowledge of, no access to. But they have access to many things about right and wrong. I think we are on the right track here in Romans 2:15, because if you look back into chapter 1 he has been teaching this very thing already. He says in Romans 1:32 that they know the ordinance of God "that those who practice such things" are worthy of death. That is exactly what he just said in 2:15. They know the ordinance of God.
"None of us lives up to the demands of his own conscience, let alone the demands that God has given." Tweet Share on Facebook
And in Romans 1:26 he says that women exchange their natural function for that which is against nature. They know it is against nature, and that is the phrase he uses in Romans 2:1415. And Romans 1:21, "They knew God." And the point of it all is to stress that every human being is guilty before God because every one of us, all human beings, "suppress the truth" (Romans 1:18), and none of us lives up to the demands of his own conscience, let alone the demands that God has given and that we know in our hearts.
So, here are the two great lessons for us that made me glad this question was asked in the first place. Real obedience, even though imperfect, real obedience to God, even though imperfect, is made possible through the work of Christ for all who believe in him. God does the decisive work of taking out the heart of rebellion and putting in the heart that loves the commandments of God: that is the glorious truth of having the law written on our hearts according to the new covenant in Hebrews 10.
Here's the second great truth: every human being in the world, every child in your family, every person that you work with, everybody in your neighborhood has an inborn knowledge of God, according to Romans 1:21, and an inborn knowledge of the moral law of God (Romans 1:32; 2:15). This means, among other things the implications are many that when we are speaking to people about the Christian faith and about why we live the way we live and what God expects of this culture and this society, we are not starting from scratch with those people. There are profound things already in their hearts that God may make use of to help them see what we are saying."
https://www.desiringgod.org/interviews/how-is-the-law-written-on-every-heart 
