Posthumously, Dallas Willard (one of my key mentors) published a book with Routledge called The Disappearance of Moral Knowledge. I learned a lot from him about the importance of tracing the history in the west of how morals became viewed as mere opinions, preferences, constructs, etc. That became the backbone of my own In Search of Moral Knowledge.
In May 2019, the Biola Center for Christian Thought convened a meeting sponsored by Dallas Willard Ministries to discuss this book. From that, I came away with an idea that there is a spiritual aspect to this issue, and not merely moral & philosophical ones. To read what I developed, see my “A Spiritual Issue with the ‘Disappearance of Moral Knowledge’” at the website for the Evangelical Philosophical Society.