Novatian is contemporaneous with Cyprian, but due to his moral rigorism on the issue of the lapsed and their readmission to communion, lead a schismatic sect from Rome. This has blotted his name in the memory of the church, though his theology, if not his ecclesiology, has been recognised as basically orthodox (certainly this seems so at Constantinople 381 and in Theodosius' edicts).
De Trinitate, written ca. 257 AD, is a fine piece of theological work, showing both the continuing tradition from Tertullian, along with several advances. It lacks the severely polemical cast of Tertullian's Against Praxeas, but nonetheless emerges in a polemical context, as Novatian himself makes plain.
Novatian begins his work with an appeal to a rule of faith, before spending 1-8 discoursing on God's transcendence. God is supreme (2), creator of all (3), good, alone, perfect (4), simple and incorruptible (5); in 5-6 Novatian is careful to understand scriptural anthropomorphism as accommodation to human understanding, and that such languages refers to God's activities.
In treating of the Son Novatian begins with OT messianic promises, before moving to treat of the two natures. (9-11) He shows rhetorical brilliance in 14-15, continually asking, “If Christ is only man, then how X, Y, Z?” He further strengthens his argument for Christ's divinity by showing the debilitation of the Spirit if Christ is not God (16). His treatment is more balanced than Tertullian, since he is at pains to establish both the humanity and divinity of Christ, not merely the Son's distinction from the Father. The absence of Tertullian's prolation Logos-Christology is notable (and welcome).
In 17-18 he, unfortunately in my opinion, follows Tertullian's line in reading OT theophanies specifically as Christophanies, by differentiated the properties of the Father and Son as unbounded/bounded, invisible/visible. Though he is right that the Son might be termed 'angel' in the meaning 'messenger', his fundamental premise here is wrong. Nonetheless, Novatian employs careful exegesis to show that Christ might be called 'angel' as 'messenger' but is not an 'angel' in the proper or substantial sense.
In treating the Sabellian position, Novatian shows logical clarity in both portraying their position (God is one, Christ is God, therefore Christ is Father), and in refuting it by distinguishing Father and Son (following Tertullian, for example, in his treatment of Jn 10:30).
Novatian's pneumatology (29) is underdeveloped, but without surprise. His brief treatment comports with his general thought. His conclusion (30-31) repeats the basic attacks of his opponents and denies them on both sides (Sabellianism and Adoptionism), while asserting the eternal existence of the Son, the unity of the Godhead, and the distinction of Persons.