Tertullian
Quintus Septimius Florens Tertullianus was a prolific early Christian writer from Carthage in North Africa. A trained lawyer, he became the first major Christian author to write in Latin, giving the Western church much of its theological vocabulary. He coined the word trinitas (Trinity) and the formula una substantia, tres personae (one substance, three persons) that would become the standard Trinitarian formula. He also developed the concept of two natures in Christ that anticipated Chalcedon. His Apologeticus defended Christians to Rome; his Against Praxeas gave the fullest pre-Nicene exposition of Trinitarian theology. Later in life he joined the Montanist movement, though his earlier orthodox writings remain invaluable.
On the Trinity
We...believe that there is one only God, but under the following dispensation, or οἰκονομία, as it is called, that this one only God has also a Son, His Word, who proceeded from Himself...This Son, then, is not other than God...The Father and the Son, while two, are one in substance.
Against Praxeas, 2 (ANF 3) (John 10:30; Matt. 28:19)
Bear always in mind that this is the rule of faith which I profess; by it I testify that the Father, and the Son, and the Spirit are inseparable from each other, and so will you know in what sense this is said. Now, observe, my assertion is that the Father is one, and the Son one, and the Spirit one, and that They are distinct from Each Other.
Against Praxeas, 9 (ANF 3) (John 14:26; John 15:26)
Thus the connection of the Father in the Son, and of the Son in the Paraclete, produces three coherent Persons, who are yet distinct One from Another. These three are one essence, not one Person, as it is said, "I and my Father are One," in respect of unity of substance not singularity of number.
Against Praxeas, 25 (ANF 3) (John 10:30; John 17:11)
On Scripture and the Rule of Faith
The rule of faith...is, that there is one only God, and that He is none other than the Creator of the world, who produced all things out of nothing through His own Word...this Word is called His Son...the Son of God...died, and was buried, according to the Scriptures, and, after He had been raised again by the Father...sent instead of Himself the Power of the Holy Ghost to lead such as believe.
On Prescription Against Heretics, 13 (ANF 3) (1 Cor. 15:3–4; John 14:16–17)
We must not appeal to the Scriptures...For it is where the essential points of the rule of faith are, that we must make our stand; for it is in the Scriptures that heretics either originate or maintain themselves. The Scriptures are indeed...the arms of heretics.
On Prescription Against Heretics, 19 (ANF 3) (2 Pet. 3:16; Acts 17:11)
On Baptism and Salvation
Happy is our sacrament of water, in that, by washing away the sins of our early blindness, we are set free and admitted into eternal life! A treatise on this matter will not be superfluous; instructing not only such as are just becoming formed [in the faith], but them who, content with having simply believed, have not examined the reasons of the traditions.
On Baptism, 1 (ANF 3) (Acts 22:16; Titus 3:5; John 3:5)
After His resurrection He promises in a pledge to His disciples that He will send them the promise of His Father; and lastly, He commands them to baptize into the Father and the Son and the Holy Ghost, not into a unipersonal God.
On Baptism, 8 (ANF 3) (Matt. 28:19; Luke 24:49)
On Christian Life and Witness
The object of our worship is the One God, He who by His commanding word, His arranging wisdom, His mighty power, brought forth from nothing this entire mass of our world, with all its array of elements, bodies, spirits, for the glory of His majesty.
Go on, rack us, torture us, condemn us, grind us to dust; your injustice is the proof that we are innocent...The oftener we are mown down by you, the more in number we grow; the blood of Christians is seed.
Apologeticus, 50.12–13 (ANF 3) (Rom. 8:36; John 12:24; Rev. 6:9)