Greggs is a local preacher in the Methodist Church and has also preached internationally as well as appearing on radio. He was the 2014 Fernley-Hartley Lecturer for the Methodist Church, and gave the 2014 Conference Lecture for the Methodist Church on the impact of doctrine on the life and ministry of the church. He serves on the World Council of Churches Faith and Order Commission and convenes the subgroup considering religious pluralism for Faith and Order. He also serves on the Faith and Order Commission of the Methodist Church. Greggs's research and writing has focused on salvation, the doctrine of God, evangelical theology, theologies of the religions, and inter-faith issues. He has published over 50 essays in theology journals including "International Journal of Systematic Theology", "Modern Theology", "Journal of Religion", "Scottish Journal of Theology", "International Journal of Public Theology", "Theology", and "Epworth Review". His books include: