In the Book of Jeremiah, the Prophet prophesied that God will make a New Covenant with His people as follows:
But this is the covenant which I will make with the house of Israel after those days,” says the LORD, “I will put My law within them, and I will write it on their hearts; and I will be their God, and they will be My people. Jeremiah 31:33
But what does "writing Laws in our hearts" mean? And how do we know if the Laws have been written? To put it simply, we do not murder or steal because there are laws governing these behaviors, and we know we will be punished if we break them. But how about when there are no such laws? If we still choose not to murder or steal even when there are no written laws, these laws have already been written in our hearts!
As Paul explained in Romans, if Gentiles who do not know God's Laws do things naturally according to the Law, this means the Law has already been written in their hearts.
For when the Gentiles without the law have a natural desire to do the things in the law, they are a law to themselves; because the work of the law is written in their hearts. Romans 2:14-15
In the Bible, it has been mentioned several times that maturity of a person is measured by his ability to discern good and evil. When we have developed this ability, and will not transgress even when there are no external Laws governing wrongful behaviors, God's Laws have already been written in our hearts, and we are ready to be accepted as His New Covenant people.