God the Father was never alone because His Word was always within Him.

For before all things God was alone—being in Himself and for Himself universe, and space, and all things. Moreover, He was alone, because there was nothing external to Him but Himself.  Yet even not then was He alone; for He had with Him that which He possessed in Himself, that is to say, His own Reason. For God is rational, and Reason was first in Him; and so all things were from Himself.  This Reason is His own Thought (or Consciousness) which the Greeks call λόγος, by which term we also designate Word or Discourse and therefore it is now usual with our people, owing to the mere simple interpretation of the term, to say that the Word was in the beginning with God; although it would be more suitable to regard Reason as the more ancient; because God had not Word from the beginning, but He had Reason even before the beginning; because also Word itself consists of Reason, which it thus proves to have been the prior existence as being its own substance. Not that this distinction is of any practical moment. For although God had not yet sent out His Word, He still had Him within Himself, both in company with and included within His very Reason, as He silently planned and arranged within Himself everything which He was afterwards about to utter through His Word. Now, whilst He was thus planning and arranging with His own Reason, He was actually causing that to become Word which He was dealing with in the way of Word or Discourse. And that you may the more readily understand this, consider first of all, from your own self, who are made “in the image and likeness of God,” for what purpose it is that you also possess reason in yourself, who are a rational creature, as being not only made by a rational Artificer, but actually animated out of His substance. Observe, then, that when you are silently conversing with yourself, this very process is carried on within you by your reason, which meets you with a word at every movement of your thought, at every impulse of your conception. Whatever you think, there is a word; whatever you conceive, there is reason.  You must needs speak it in your mind; and while you are speaking, you admit speech as an interlocutor with you, involved in which there is this very reason, whereby, while in thought you are holding converse with your word, you are (by reciprocal action) producing thought by means of that converse with your word. Thus, in a certain sense, the word is a second person within you, through which in thinking you utter speech, and through which also, (by reciprocity of process,) in uttering speech you generate thought. The word is itself a different thing from yourself. Now how much more fully is all this transacted in God, whose image and likeness even you are regarded as being, inasmuch as He has reason within Himself even while He is silent, and involved in that Reason His Word!

Tertullian
Against Praxeas
Chapter V

The Son is of the same substance and the Spirit proceeds from the Father through the Son

But as for me, who derive the Son from no other source but from the substance of the Father, and (represent Him) as doing nothing without the Father’s will, and as having received all power from the Father, how can I be possibly destroying the Monarchy from the faith, when I preserve it in the Son just as it was committed to Him by the Father? The same remark (I wish also to be formally) made by me with respect to the third degree in the Godhead, because I believe the Spirit to proceed from no other source than from the Father through the Son. Look to it then, that it be not you rather who are destroying the Monarchy, when you overthrow the arrangement and dispensation of it, which has been constituted in just as many names as it has pleased God to employ.

Tertullian
Against Praxeas
Chapter IV

The Son and the Holy Spirit are the same substance as the Father

Therefore, inasmuch as the Divine Monarchy also is administered by so many legions and hosts of angels, according as it is written, “Thousand thousands ministered unto Him, and ten thousand times ten thousand stood before Him;” and since it has not from this circumstance ceased to be the rule of one (so as no longer to be a monarchy), because it is administered by so many thousands of powers; how comes it to pass that God should be thought to suffer division and severance in the Son and in the Holy Ghost, who have the second and the third places assigned to them, and who are so closely joined with the Father in His substance, when He suffers no such (division and severance) in the multitude of so many angels? Do you really suppose that Those, who are naturally members of the Father’s own substance, pledges of His love, instruments of His might, nay, His power itself and the entire system of His monarchy, are the overthrow and destruction thereof? You are not right in so thinking.

Tertullian
Against Praxeas
Chapter III

The pagans struggle believing in one God, and see the three persons as three gods

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 οἰκονομία . The numerical order and distribution of the Trinity they assume to be a division of the Unity; whereas the Unity which derives the Trinity out of its own self is so far from being destroyed, that it is actually supported by it.

Tertullian
Against Praxeas
Chapter III

The Trinity is one God in substance but three persons

But keeping this prescriptive rule inviolate, still some opportunity must be given for reviewing (the statements of heretics), with a view to the instruction and protection of divers persons; were it only that it may not seem that each perversion of the truth is condemned without examination, and simply prejudged; especially in the case of this heresy, which supposes itself to possess the pure truth, in thinking that one cannot believe in One Only God in any other way than by saying that the Father, the Son, and the Holy Ghost are the very selfsame Person. As if in this way also one were not All, in that All are of One, by unity (that is) of substance; while the mystery of the dispensation is still guarded, which distributes the Unity into a Trinity, placing in their order the three Persons—the Father, the Son, and the Holy Ghost: three, however, not in condition, but in degree; not in substance, but in form; not in power, but in aspect; yet of one substance, and of one condition, and of one power, inasmuch as He is one God, from whom these degrees and forms and aspects are reckoned, under the name of the Father, and of the Son, and of the Holy Ghost. How they are susceptible of number without division, will be shown as our treatise proceeds.

Tertullian
Against Praxeas
Chapter II

A summation of the orthodox faith

We, however, as we indeed always have done (and more especially since we have been better instructed by the Paraclete, who leads men indeed into all truth), 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, by whom all things were made, and without whom nothing was made. Him we believe to have been sent by the Father into the Virgin, and to have been born of her—being both Man and God, the Son of Man and the Son of God, and to have been called by the name of Jesus Christ; we believe Him to have suffered, died, and been buried, according to the Scriptures, and, after He had been raised again by the Father and taken back to heaven, to be sitting at the right hand of the Father, and that He will come to judge the quick and the dead; who sent also from heaven from the Father, according to His own promise, the Holy Ghost, the Paraclete, the sanctifier of the faith of those who believe in the Father, and in the Son, and in the Holy Ghost. That this rule of faith has come down to us from the beginning of the gospel, even before any of the older heretics, much more before Praxeas, a pretender of yesterday, will be apparent both from the lateness of date which marks all heresies, and also from the absolutely novel character of our new-fangled Praxeas.

Tertullian
Against Praxeas
Chapter I

God the Word created flesh as an image of his coming in the flesh later in time

For, whatever was the form and expression which was then given to the clay (by the Creator) Christ was in His thoughts as one day to become man, because the Word, too, was to be both clay and flesh, even as the earth was then.  For so did the Father previously say to the Son: “Let us make man in our own image, after our likeness.” And God made man, that is to say, the creature which He moulded and fashioned; after the image of God (in other words, of Christ) did He make him. And the Word was God also, who being in the image of God, “thought it not robbery to be equal to God.” Thus, that clay which was even then putting on the image of Christ, who was to come in the flesh, was not only the work, but also the pledge and surety, of God.

Tertullian
On the Resurrection of the Flesh
Chapter VI

Christ is the Word, the Spirit and the Power of God

The expression is in the singular number, as referring to the Lord, “He was born of God.”  And very properly, because Christ is the Word of God, and with the Word the Spirit of God, and by the Spirit the Power of God, and whatsoever else appertains to God.

Tertullian
On the Flesh of Christ
Chapter XIX

It was God who became man to save us, not an angel

There is, then, no longer but one God, but one Savior, if there be two to contrive salvation, and one of them in need of the other. But was it His object indeed to deliver man by an angel? Why, then, come down to do that which He was about to expedite with an angel’s help? If by an angel’s aid, why come Himself also? If He meant to do all by Himself, why have an angel too? He has been, it is true, called “the Angel of great counsel,” that is, a messenger, by a term expressive of official function, not of nature. For He had to announce to the world the mighty purpose of the Father, even that which ordained the restoration of man.  But He is not on this account to be regarded as an angel, as a Gabriel or a Michael. For the Lord of the Vineyard sends even His Son to the laborers to require fruit, as well as His servants.

…Forasmuch, however, as it has been declared concerning the Son Himself, “Thou hast made Him a little lower than the angels” how will it appear that He put on the nature of angels if He was made lower than the angels, having become man, with flesh and soul as the Son of man? As “the Spirit of God,” however, and “the Power of the Highest,” can He be regarded as lower than the angels,—He who is verily God, and the Son of God? Well, but as bearing human nature, He is so far made inferior to the angels; but as bearing angelic nature, He to the same degree loses that inferiority. This opinion will be very suitable for Ebion, who holds Jesus to be a mere man, and nothing more than a descendant of David, and not also the Son of God; although He is, to be sure, in one respect more glorious than the prophets, inasmuch as he declares that there was an angel in Him, just as there was in Zechariah. Only it was never said by Christ, “And the angel, which spake within me, said unto me.” Neither, indeed, was ever used by Christ that familiar phrase of all the prophets, “Thus saith the Lord.” For He was Himself the Lord, who openly spake by His own authority, prefacing His words with the formula, “Verily, verily, I say unto you.” What need is there of further argument? Hear what Isaiah says in emphatic words, “It was no angel, nor deputy, but the Lord Himself who saved them.”

Tertullian
On the Flesh of Christ
Chapter XIV