The nature of the hypostatic union — God and man

Christ could not be described as being man without flesh, nor the Son of man without any human parent; just as He is not God without the Spirit of God, nor the Son of God without having God for His father. Thus the nature of the two substances displayed Him as man and God,—in one respect born, in the other unborn; in one respect fleshly, in the other spiritual; in one sense weak, in the other exceeding strong; in one sense dying, in the other living. This property of the two states—the divine and the human—is distinctly asserted with equal truth of both natures alike, with the same belief both in respect of the Spirit and of the flesh. The powers of the Spirit, proved Him to be God, His sufferings attested the flesh of man.

Tertullian
On the Flesh of Christ
Chapter V

The Logos is begotten from within God, not outside of him

Indeed, as soon as He perceived It to be necessary for His creation of the world, He immediately creates It, and generates It in Himself. “The Lord,” says the Scripture, “possessed me, the beginning of His ways for the creation of His works. Before the worlds He founded me; before He made the earth, before the mountains were settled in their places; moreover, before the hills He generated me, and prior to the depths was I begotten.” Let Hermogenes then confess that the very Wisdom of God is declared to be born and created, for the especial reason that we should not suppose that there is any other being than God alone who is unbegotten and uncreated. For if that, which from its being inherent in the Lord was of Him and in Him, was yet not without a beginning,—I mean His wisdom, which was then born and created, when in the thought of God It began to assume motion for the arrangement of His creative works,—how much more impossible is it that anything should have been without a beginning which was extrinsic to the Lord!

Tertullian
Against Hermogenes
Chapter XVIII

There is only one God and Father of all

For “although there be that are called gods” in name, “whether in heaven or in earth, yet to us there is but one God the Father, of whom are all things;” whence the greater reason why, in our view, that which is the property of God ought to be regarded as pertaining to God alone, and why (as I have already said) that should cease to be such a property, when it is shared by another being. Now, since He is God, it must necessarily be a unique mark of this quality, that it be confined to One. Else, what will be unique and singular, if that is not which has nothing equal to it? What will be principal, if that is not which is above all things, before all things, and from which all things proceed? By possessing these He is God alone, and by His sole possession of them He is One.  If another also shared in the possession, there would then be as many gods as there were possessors of these attributes of God.

Tertullian
Against Hermogenes
Chapter IV

The Nature of the Logos

The name Word bestowed upon the Son of God reveals better than any other name the mystery of the inner relationship between the First and Second Persons of the Holy Trinity, God the Father and God the Son. A thought and a word are distinct from each other in that the thought dwells in the mind, whereas the word is the expression of the thought; yet the two are inseparable. The thought does not exist without the word, nor does the word without the thought. A thought is like a word which is concealed within, and a word is that which gives expression to the thought. The thought takes the form of a word to convey the content of the thought to its hearers. Looked at in this way, the thought, being an independent principle, is the father of the word, and the word is the son of the thought. The word cannot exist prior to the thought, yet it does not originate from without; it comes from the thought and remains inseparable from the thought. Similarly, the Father, the supreme and all-encompassing Thought, produced from His bosom the Son, the Word, His first Interpreter and Herald.

St. Dionysius of Alexandria
The Logos, 230-265 A.D.

Christ and the Spirit are one substance with the Creator

For when you do not deny that the Creator’s Son and Spirit and Substance is also His Christ, you must needs allow that those who have not acknowledged the Father have failed likewise to acknowledge the Son through the identity of their natural substance; for if in Its fulness It has baffled man’s understanding, much more has a portion of It, especially when partaking of the fulness.

Tertullian
The Five Books Against Marcion, Book III
Chapter VI

The Spirit of God is God

We must at the outset hold fast the meaning of the Greek scripture, which has afflatus, not spirit. Some interpreters of the Greek, without reflecting on the difference of the words, and careless about their exact meaning, put spirit for afflatus; they thus afford to heretics an opportunity of tarnishing the Spirit of God, that is to say, God Himself, with default.

Tertullian
The Five Books Against Marcion, Book II
Chapter IX

The doctrine of God came from the Apostles

Some disputed about eating idol sacrifices, others about the veiled dress of women, others again about marriage and divorce, and some even about the hope of the resurrection; but about God no one disputed. Now, if this question also had entered into dispute, surely it would be found in the apostle, and that too as a great and vital point. No doubt, after the time of the apostles, the truth respecting the belief of God suffered corruption, but it is equally certain that during the life of the apostles their teaching on this great article did not suffer at all; so that no other teaching will have the right of being received as apostolic than that which is at the present day proclaimed in the churches of apostolic foundation.

…Show us, then, one of your churches, tracing its descent from an apostle, and you will have gained the day.

Tertullian
The Five Books Against Marcion, Book I
Chapter XXI