In Tertullian's book, he wrote, for example, that:
The simple, indeed, (I will not call them unwise and unlearned,) who always constitute the majority of believers, are startled at the dispensation (of the Three in One), on the ground that their very rule of faith withdraws them from the world's plurality of gods to the one only true God; not understanding that, although He is the one only God, He must yet be believed in with His own dispensation. The numerical order and distribution of the Trinity they assume to be a division of the Unity.
Tertullian was also the first Christian to deal specifically with the relation of the two natures in Christ. How, he asked, could the divine Word "become" flesh? Not, he asserted, by transforming himself into flesh, because then he would no longer be divine. Rather, he put on flesh; thus, the divine "substance" and the human "substance" both constitute the one "person" of Christ.
Like the Apologists, Tertullian posited a two-stage existence in the Word: First as immanent within the Father, then as expressed at the Son's generation:
There are some who allege that even Genesis opens thus in Hebrew: "In the beginning God made for Himself a Son." As there is no ground for this, I am led to other arguments derived from God's own dispensation, in which He existed before the creation of the world, up to the generation of the Son. For before all things God was alone - being in Himself and for Himself