Work Of The Law - Romans 2:14-16

Romans 2:14-16 ESV 

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 on that day when, according to my gospel, God judges the secrets of men by Christ Jesus.

It is the Holy Spirit Who provides the work of the law to be written on their hearts. It is a new nature that awakens the conscience and bears witness to the presence of God. It is that presence that allows us to know wrong from right. It is the presence and work of the Spirit which proves our salvation.