Here are some highlights from St. John of Damascus’ On the Orthodox Faith, 1.8 (read it here for yourself!). Each paragraph is a separate quotation.
In treating, then, of the generation of the Son, it is an act of impiety to say that time comes into play and that the existence of the Son is of later origin than the Father. For we hold that it is from Him, that is, from the Father’s nature, that the Son is generated. And unless we grant that the Son co-existed from the beginning with the Father, by Whom He was begotten, we introduce change into the Father’s subsistence, because, not being the Father, He subsequently became the Father. For the creation, even though it originated later, is nevertheless not derived from the essence of God, but is brought into existence out of nothing by His will and power, and change does not touch God’s nature. For generation means that the begetter produces out of his essence offspring similar in essence. But creation and making mean that the creator and maker produces from that which is external, and not out of his own essence, a creation of an absolutely dissimilar nature.
But if we say that the Father is the origin of the Son and greater than the Son, we do not suggest any precedence in time or superiority in nature of the Father over the Son (for through His agency He made the ages), or superiority in any other respect save causation. And we mean by this, that the Son is begotten of the Father and not the Father of the Son, and that the Father naturally is the cause of the Son: just as we say in the same way not that fire proceedeth from light, but rather light from fire.
it is quite impossible to find in creation an image that will illustrate in itself exactly in all details the nature of the Holy Trinity. For how could that which is create and compound, subject to flux and change, circumscribed, formed and corruptible, clearly shew forth the super-essential divine essence, unaffected as it is in any of these ways? Now it is evident that all creation is liable to most of these affections, and all from its very nature is subject to corruption.
But the Son is derived from the Father after the manner of generation, and the Holy Spirit likewise is derived from the Father, yet not after the manner of generation, but after that of procession. And we have learned that there is a difference between generation and procession, but the nature of that difference we in no wise understand. Further, the generation of the Son from the Father and the procession of the Holy Spirit are simultaneous.
The subsistences then we say are perfect, that we may not conceive of the divine nature as compound. For compoundness is the beginning of separation. And again we speak of the three subsistences as being in each other, that we may not introduce a crowd and multitude of Gods. Owing to the three subsistences, there is no compoundness or confusion: while, owing to their having the same essence and dwelling in one another, and being the same in will, and energy, and power, and authority, and movement, so to speak, we recognise the indivisibility and the unity of God. For verily there is one God, and His word and Spirit.
For there is one essence, one goodness, one power, one will, one energy, one authority, one and the same, I repeat, not three resembling each other. But the three subsistences have one and the same movement.
the subsistences dwell in one another, in no wise confused but cleaving together, according to the word of the Lord, I am in the father, and the father in Me: nor can one admit difference in will or judgment or energy or power or anything else whatsoever which may produce actual and absolute separation in our case. Wherefore we do not speak of three Gods, the Father, the Son, and the Holy Spirit, but rather of one God, the holy Trinity, the Son and Spirit being referred to one cause, and not compounded or coalesced according to the synæresis of Sabellius.