Written Into Man's Heart
It was written into man’s heart in the garden of Eden as his best friend. Before he had Eve, he had the law of God written in his heart, which is why what the Spirit does in the fulfillment of the promise of the new covenant is to write into our hearts…the law (Jer. 31:33). Twice in the New Testament Jeremiah’s prophecy of the law being written on our hearts in the new covenant is seen to be specifically fulfilled in the gospel (Heb. 8:10; 10:16).
Sinclair Ferguson, "Life and Ministry", in Joel Beeke and Greg Salazar, eds., William Perkins: Architect of Puritanism (Grand Rapids, MI: Reformation Heritage Books, 2019).