In Romans 2, the apostle Paul tells us that moral knowledge of God’s law is innate to man. He writes:

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. 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 2:14-15

Paul’s point is that human beings have God’s law written in their hearts. Every human being knows what God requires of them because these requirements are inside them by nature. We should conclude three things from this truth.
1. Right and wrong is not so much taught as it is innate. There is a tendency to believe that people do not already know right and wrong and they must be instructed in it. Moral knowledge must be poured in them from the outside. Paul says that this is wrong. God’s law is written in our hearts and this means that knowledge of right and wrong is innate.

Like dirt on a window, sin obscures a clear knowledge of the law. This is why moral instruction is necessary, not so much to implant God’s law into us, as to help us see what is there more clearly. Moral instruction is also necessary in order to help us develop virtues. However, every human being from birth knows God’s law and is therefore morally responsible.  
2. Doing right is being human, doing wrong is being less human. The knowledge of God’s law in our hearts is part of the image of God in which we were created. The image of God is what makes us human. Animals are not created in this image. Therefore, to be truly human, we need to obey Gods law. Sin actually makes us less human. When we sin we are denying or suppressing what we know to be our true human nature. This is why many of the great saints of the past made a connection between virtue and happiness. Serving and obeying God is the greatest good or summum bonum. It is the goal and purpose for which we were created, our telos. We were created to be happy creatures, but that happiness only comes when we are doing what we were designed to do, namely serve and obey God.    

3. Sin ruined everything. Because of sin we do not and cannot perfectly obey God’s law. Misery and unhappiness are now part of man because of sin. This does not negate our moral responsibility. The law is still written in our hearts and God holds us accountable to that law. Apart from Christ the law that was to guide us into happiness. The law is now the basis for our just condemnation by God. Since we violate God’s law from birth countless times, we are guilty. That guilt must be punished by God’s wrath. We will either pay that penalty ourselves or trust in Jesus Christ who paid it in our place on the cross. The law written in our hearts ought to drive us to Christ. It ought to show us that we fall short of God’s requirements and this ought to cause us to look for salvation outside of ourselves. That salvation is freely available in Jesus Christ alone. He is the only one that went to the cross to pay the guilt of our sins. Faith in Christ is the only way to salvation.