Simple answer,
We fulfill it through Christ.
The law that He has written upon our hearts.
The law by Which we by conscience fulfill.
Romans 2:14-16 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;) In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.