Institutes of the Christian Religion (1559) - John Calvin
Book 2 - God the Redeemer
Chapter 11 - The Difference between the Two Testaments
Sections 7-8 - The OT is literal; the NT is spiritual
Section 7 - Biblical origin and meaning of this difference
Jer 31:31-34; 2 Cor 3:6-11
The OT is literal, written on stone, merely commanding and rewarding obedience (forbidding and punishing disobedience), and not able "to change or correct the depravity of heart," as the spiritual NT can.
Section 8 - The difference in detail, according to 2 Cor 3
The NT is spiritual, because God wrote it on our hearts - 2 Cor 3:6.
The cermonial law had to "die & vanish" while the NT lasts forever - 3:10-11
It was weak in itself, while the moral law's weakness was in the people's sin, not in itself.