Would it not be correct to say that The Father and the Holy Ghost co-beget the Son? This is what the Creed seems to say in "by the power of the Holy Spirit He was born of the Virgin Mary".
Or am I confusing the incarnation and the trinitarian generation to an intolerable level again?
Yes! Who is God? And What is God? God is the eternal Father. But to be a Father means to be in relationship. With Whom? With His Son Whom He eternally begets. Scripture says “the only-begotten Son” . In fact, there can be only one begotten Son of the Father because this begetting is Absolute and Perfect generation.
Think of it this way, in spiritual life in nature we see that there are two “activities” of our faculties: intellect and will. Because nature images God Who is it’s author, we weakly deduce that God Who is pure Spirit and infinite Substance fulfills both these activities in Infinite Eternity.
The only begotten Son is proceeds eternally form the Divine Intellect. But the Divine Intellect knows all Eternity which is none other than God Himself. Therefore, throughout Eternity the Son is generated by the Father whereby the Father communicates His entire, identical substance to the Son.
Speaking about the natural order, if through intellectual operations a concept is begotten which is the image of the object understood, then in volitional activities an act of love causes an inclination towards the loved one.
Apply the Divine Will to the relationship of the Father and the Son, Who both share completely and identically one and the same substance, but have only different relations towards each other. The “spiration” between them as an act of Love in the Divine Will, complete and Absolute, causes the eternal procession of the Holy Spirit.
Confusing enough? That’s why it is THE mystery of the Catholic Faith, Divinely revealed by Christ.
The line in the Creed concerning the birth of Christ by the power of the Holy Spirit refers to the Uncreated Eternal Son’s conception in His Created order (the world) by Mary as His true mother within context of His Incarnation (i.e. His becoming truly a unique human, without changing His Eternal Origin or Substance from His Father – the 2nd great mystery of the Catholic Faith) into His Creation.
Um, I’ll send this now and see how it does;)
Or am I confusing the incarnation and the trinitarian generation to an intolerable level again?
I think so. The Incarnation comes after the Trinity. The relation between the Father and Son is generation, or paternity and filiation. The relation between the Father and Son, and the Holy Spirit is procession or spiration.