What do the Apologists mean by three persons as "one in nature" if the Son and Spirit are not eternal? The christology of the Apologists is known to be the Logos Christology. Their teaching is summarized as follows: 


Logos endiathetos - eternal reason within God
Logos prophorikos - the spoken word of God ( the begotten Son)


The Son, according to the Apologists, was begotten in order to create all things. It means that there is neither eternal Father nor eternal Son. (source)

The Apologists believe that God became a Father when his logos became his Son. Although they are two persons, there is still one God because the Son is from God's being and hence, inseparable to him. They further teach that the Father and the Son are "of same nature" and even used analogies like the Sun and its light are of same nature. The Latin Apologists, Tertullian says:


  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, John 10:30 in respect of unity of
  substance not singularity of number. Against Praxeas 25


On the otherhand, Athanasius taught that the Father and the Son are both eternal. He taught that the Son is without beginning because the Father who begot him is without beginning. (source)

So as I understand it we have:


Athanasius ( Son = eternal) of one nature with the Father. I do believe this is logical.
Apologists (Son = not eternal) of one nature with the Father. I can't understand how could they affirm the Son's consubstantiality with the Father when they affirm that the Son has a beginning of existence.


Now, my question is this:

In what sense do the Apologists mean when they affirm the consubstantiality of the Trinity? Both the Apologists and Athanasius ( + all of the Pro-Nicene Fathers) agree that the Three are "of one nature, essence, substance, being", but the former affirm that the Son has a beginning while the latter affirm that the Son has no beginning, so what's the meaning of their affirmation "of one nature"?
Not all the Apologists taught the Trinity using the Stoic λόγος and not all of them have equal understanding of the divine substance. 

The Following are facts about the belief of an Apologist who uses the Stoic λόγος in teaching the Trinity:

1) The λόγος is eternal just as God who has it is eternal. It means that λόγος is coeval with God.

2) In the beginning [ of creation]or before creation, God made his λόγος a subsisting person to be his Son.It means that the Son isn't eternal.

3) The begetting of the λόγος is not necessary but only happened because of God's will in creating all things and in saving people.

This sort of teaching is known as the "economic Trinitarianism."

The Latin Apologist Tertullian taught that the Son learned things in order to prepare for the incarnation and so that people might also be prepared for his incarnation( Praxeas 16:3). 

This sort of theology makes the Son subordinate to the Father in its real sense of Father and Son relationship but also stretches into the divine life itself. What this means is that the Father does not lack any knowledge because he his λόγος in all his activities ad extra and hence, by having the Son, experience is not necessary for him but for his λόγος alone.

Notice the analogy used of Tertullian. He taught that the Sun cannot be seen except by its light and so is the Father by the Son ( cf:John 1:18).

The only eternal is the 'name' of God and 'God' himself as well as his substance. 

The λόγος, being in God, is eternal but as proceeding from God he is still of same substance, having been of same origin but now, with God's name and subsistence as the Son. 

Athenagoras ( AD. 133-190) is an Apologist yet he did not subscribe to the Stoic λόγος in teaching the Trinity:

If … it occurs to you to inquire what is meant by the Son, I will state briefly that he is the first product of the Father, not as having been brought into existence, for from the beginning God, who is the eternal mind, had the Logos in himself, being from eternity instinct with Logos. ( Plea for the Christians, Chap. 10)

Irenaeus (AD. 120-200)too is an Apologist yet he believed in an eternal Son:

180 AD Irenaeus "But the Son, eternally co-existing with the Father, from of old, yea, from the beginning, always reveals the Father to Angels, Archangels, Powers, Virtues..." (Against Heresies, Book II, ch. 30, section 9)

Also, Clement of Alexnadria ( AD. 150-215) teaches the same thing, that is, that the Son is eternal: 

When [John] says, "from the beginning," the elder explained to this effect, that the beginning of generation is not separated from the beginning of the Creator. For when he says, "That which is from the beginning," he touches on the generation without beginning of the Son, who is co-existent with the Father. There was, then, a Word importing an unbeginning eternity, as also the Word itself, that is, the Son of God, who being by equality of substance one with the Father, is eternal and uncreated.(Fragments of Clement of Alexandria: Comments on the First Epistle of John from The Ante-Nicene Fathers, vol. II)

These received the teachings of the Apostles about theology in its purest form and in the next century ( 3rd century) , we see this continuation of teaching an eternal Trinity as opposed to the economic Trinity, by Gregory of Thaumaturgus, Novatian and Dionysius of Rome and in the next century( 4th century), Alexander of Alexandria and Athanasius ( + the Cappadocian fathers) all defended this ancient teaching of an eternal Trinity against the error of some of the Apologists that resulted into Dynamic Monarchianism and Arianism. 

Conclusion: 

The theological speculation of some of the Apologists were clearly an error. The adoption of the Stoic λόγος , therefore, is not a wise thing to do.