Parsons, Stuart E. “Very Early Trinitarian Expressions.” Tyndale Bulletin 65.1 (2014): 141–152.
While older scholarship identified the earliest use of Trinitarian terminology near the end of the second century in the work of Theophilus of Antioch, some recent studies have challenged this view. However, while affirming certain insights of these newer studies, it is necessary to revisit them in light of the historical setting of the second-century apologists. In reality, Theophilus and other early apologists evidenced a certain implicit Trinitarianism by affirming unity, distinction, eternal pre-existence and economic subordination in the Godhead. Studies of early Trinitarian terminology must look beyond explicit descriptions of the Godhead. They must consider also broad patterns of implicit Trinitarianism.
Tyndale Bulletin (website: http://www.TyndaleHouse.com/Bulletin/65=2014/10_Parsons_12.pdf)