Romans 2:12, 14-15

When the Gentiles sin, they will be destroyed, even though they never had God’s written law. And the Jews, who do have God’s law, will be judged by that law when they fail to obey it.

Even Gentiles, who do not have God’s written law, show that they know his law when they instinctively obey it, even without having heard it. They demonstrate that God’s law is written in their hearts, for their own conscience and thoughts either accuse them or tell them they are doing right.