See, for example, Mark J. Edwards, “Gnostics and Valentinians in the Church Fathers,” Journal of Theological Studies (JTS) 40 (1989), pp. 26‒47; Mark J. Edwards, “Neglected Texts in the Study of Gnosticism,” JTS 41 (1990), pp. 26‒50; Bentley Layton, “Prolegomena to the Study of Gnosticism,” in L.M. White & O.L. Yarbrough, eds., The Social World of the First Christians: Essays in Honor of Wayne Meeks (Minneapolis: Augsburg Fortress, 1995), pp. 334‒350; David Brakke, The Gnostics: Myth, Ritual, and Diversity in Early Christianity (Cambridge, MA: Harvard Univ. Press, 2012).