Dr. Ngien is the author of several monographs including The Suffering of God according to Martin Luther's Theologia Crucis (Peter Lang, 1995); Apologetic for Filioque in Medieval Theology (Paternoster, 2005); Luther as a Spiritual Adviser: The Interface of Theology and Piety in Luther's Devotional Writings (Paternoster, 2007); Gifted Response: The Triune God as the Causative Agency of our Responsive Worship (Paternoster, 2008), Fruit for the Soul: Luther on the Lament Psalms (Fortress Press, 2015), and Luther's Theology of the Cross: Christ in Luther's Sermons on John (Cascade Books, 2018). He edited McGrath Festschrit, The Interface of Science, Theology, and Religion: Essays in Honor of Alister E. McGrath (Oregon: Wipf & Stock, 2019).

His articles have appeared in journals such as International Journal of Systematic Theology, Scottish Bulletin of Evangelical Theology, Evangelical Review of Theology, Heythrop Journal, etc. His article, "The God Who Suffers", appeared on Christianity Today's 50th anniversary webpage, and is considered one of the significant works.

His book, Reformation Spirituality: The Unity of Theology and Practice in Luther and Calvin, will be released next year (Baker Academic, 2021). P