St. Maximus the Confessor, a prominent Greek Father of the Church (d. 662), in his Questiones ad Thalassium (q.60; PG 90; 620-621) comments on God’s foreknowledge of Christ: “Foreknown, indeed, before the foundation of the world, He has been manifested in the last times for your sakes” (1 Pt 1:20).

Below are the pertinent passages (if you are interested in reading more of his Christology I would highly recommend On the Cosmic Mystery of Christ which provides translations from St Maximus’ two main collections of theological reflections, his Ambigua and his Questions to Thalassius, plus one of his Christological opuscula).

From the pen of St. Maximus the Confessor…

The scriptural text calls the mystery of Christ “Christ.” The great Apostle clearly testifies to this when he speaks of the mystery hidden from the ages, having now been manifested (Col 1:26). He is of course referring to Christ, the whole mystery of Christ, which is manifestly the ineffable and incomprehensible hypostatic union between Christ’s divinity and humanity…

This is the great and hidden mystery, at once the blessed end for which all things are ordained. It is the divine purpose conceived before the beginning of created beings. In defining it we would say that this mystery is the preconceived goal for which everything exists, but which itself exists on account of nothing. With a clear view to this end, God created the essences of created beings, and such is, properly speaking, the terminus of His providence and of the things under His providential care. Inasmuch as it leads to God, it is the recapitulation of the things he has created. It is the mystery which circumscribes all the ages, and which reveals the grand plan of God (cf. Eph 1:10-11), a super-infinite plan infinitely preexisting the ages. The Logos, by essence God, became a messenger of this plan (cf. Is 9:5) when He became a man and, if I may rightly say so, established Himself as the innermost depth of the Father’s goodness while also displaying in Himself the very goal for which His creatures manifestly received the beginning of their existence.

Because of Christ – or rather, the whole mystery of Christ – all the ages of time and the beings within those ages have received their beginning and end in Christ. For the union between a limit of ages and limitlessness, between measure and immeasurability, between finitude and infinity, between Creator and creation, between rest and motion, was conceived before the ages. This union has been manifested in Christ at the end of time and in itself brings God’s foreknowledge to fulfillment, in order that naturally mobile creatures might secure themselves around God’s total and essential immobility, desisting altogether from their movement toward themselves and toward each other. The union has been manifested so that they might also acquire, by experience, an active knowledge of Him in whom they were made worthy to find their stability and to have abiding unchangeably in them the enjoyment of this knowledge…

This mystery was known solely to the Father, the Son, and the Holy Spirit before all the ages. It was know to the Father by His approval (ενδοκια), to the Son by His carrying it out (αυτουρια), and to the Holy Spirit by His cooperation (συνεργεια) in it. For there is one knowledge shared by the Father, the Son, and the Holy Spirit because They also share one essence and power. The Father and the Holy Spirit were not ignorant of the Incarnation of the Son because the whole Father is by essence in the whole Son who Himself carried out the mystery of our salvation through His Incarnation. The Father Himself did not become incarnate but rather approved the Incarnation of the Son. Moreover, the whole Holy Spirit exists by essence in the whole Son, but He too did not become incarnate but rather cooperated in the Son’s ineffable Incarnation for our sake. Whether, then, one speaks of “Christ” or the “mystery of Christ,” the Holy Trinity alone – Father, Son, and Holy Spirit – foreknew it. And no one should question how Christ, who is one of the Holy Trinity, was foreknown by the Trinity, when recognizing that Christ was foreknown not as God but as man. In other words, it was His Incarnation for humanity’s sake in the economy of salvation that was foreknown. For that which is eternal and forever transcending cause and reason could never be foreknown. Foreknowledge is of being who have a beginning of existence because they have a cause.

Thus Christ was foreknown not as what He was in Himself by nature but as what He manifested when, in the economy of salvation, He subsequently became human on our behalf. For truly He who is the Creator of the essence of created beings by nature had also to become the very Author of the deification of creatures by grace, in order that the Giver of well-being might appear also as the gracious Giver of eternal well-being. Since, therefore, no created being knows what itself of any other being absolutely is in its essence, it only follows that no created being by nature has foreknowledge of any future beings. Only God, who transcends created beings, and who knows what He Himself is in essence, foreknows the existence of all His creatures even before their creation. And in the future He will by grace confer on those created beings the knowledge of what they themselves and other beings are in essence, and manifest the principles of their origin which preexist uniformly in Him.

Indeed, we reject the argument of some who say that Christ was foreknown before the foundation of the world to those to whom He was later manifested at the end of time, as though those beings were themselves present with the foreknown Christ before the foundation of the world, and as though the scriptural Word were running awry from the truth and suggesting that the essence of rational beings is coeternal with God. For it is impossible to be completely coexistent with Christ, just as it is furthermore impossible ever to depart from Him entirely, since the termination of time is fixed within Christ, as the stability (στασις) of mobile created beings, as stability wherein no created being will know any change at all.