Question: What is the "hypostatic union"? It there anything about it in the Bible? In the Fathers of the Church? In the prayers of the Church?
Answer: Without getting into a complex philosophical discussion, we can say that the Greek word hypostasis (ύπόστασις) refers to the “underlying basis” of something. As a theological term hypostasis has taken on progressively sharper meaning, eventually coming to include all that that makes up a rational person. The term "hypostatic union" is used to include everything – both human and divine -- that makes up the Person of Jesus Christ. As God, the Second Person of the Blessed Trinity existed from all eternity. The hypostatic union began only with His Incarnation, as He assumed human nature, uniting His divinity with humanity, being conceived by the Holy Ghost in the womb of the Blessed Virgin Mary. From that moment in time it is possible to speak of "Jesus Christ, true God and true man." The incarnation is in fulfillment of the promise made by God to send a Redeemer able to set right the offence of Adam.
The Incarnation is described in some detail in the Gospels of Saints Luke and John. Luke’s account (chapter 1) is written from the standpoint of the Virgin Mary, describing her conception in somewhat poetic terms: "Thou shalt conceive in thy womb and shalt bring forth a Son; and thou shalt call His name Jesus…. The Holy Ghost shall come upon thee, and the power of the Most High shall overshadow thee; and therefore the Holy One to be born of thee shall be called the Son of God."
Saint John’s Gospel is more theologically precise. It is written with the terminology of Greek philosophy, in which the "Logos" ("Verbum" in Latin or "Word" in English) is the eternal principle which gives order to the Universe. John (chapter 1) refers to the Second Person of the Trinity as "the Word": "In the beginning was the Word, and the Word was with God; and the Word was God…. All things were made through Him…. And the Word became flesh and dwelt amongst us.
That Jesus Christ is God and the Son of God, equal to the Father is corroborated in a number of passages: for example, Mt. 16, in which Peter says, "Thou art the Christ the Son of the living God," and our Lord replies, "flesh and blood has not revealed this to thee, but My Father in heaven"; or John 10, in which Jesus proclaims, "I and the Father are one"; or in chapter 14 in which He asks, "Dost thou not believe that I am in the Father, and the Father in Me." The Jews certainly understood that our Lord was claiming to be equal to God – they almost stoned Him to death for claiming that (Jn.8): "Amen, amen, I say unto you, Before Abraham was, I am" – and did, in fact, deliver Him over to the Romans to be put to death for precisely that (Jn. 19): "We have a law, and according to that Law He must die, because He has made Himself Son of God."
The Scriptures also take pains to relate that Jesus was truly a man, and not some sort of spirit projection without material reality. He eats and drinks with the apostles and others, and sometimes prepares the food. He bleeds real blood, and experiences fear and pain and thirst. Saint John, who laid his head on our Lord’s breast at the Last supper, speaks of Him as being tangible – as He was at the Last Supper, and is in every re-presentation of that Supper, under the appearances of bread and wine at every offering of Holy Mass. It is a sublime combination of the material and the spiritual that our Lord would promise (John 6) eternal life to those who eat His body and drink His blood.
The early Church vigorously resisted any theory that contradicted the unity of the true God with the true man in the Person of Jesus Christ. We have various creedal statements made by the Church in ecumenical council, and a number of papal pronouncements, phrased to present the authentic doctrine in response to heretical theories. Among the best known are the Nicene Creed of 325 AD, its revised form of 381 AD known as the Nicene-Constantinopolitan Creed, and the Athanasian Creed of the fourth or fifth century. Among the more influential defenders of the Faith were Pope Saint Callistus, c. 217, (against Sabellius, who taught a sort of evolution of the Father into the Son and then into the Holy Ghost); St. Athanasius of Alexandria, c. 325, (against Arius, who taught that the Second Person was a created being); Saint Basil and Pope Saint Damasus, c. 375, (against Apollinaris of Laodicea, who imagined Christ as God with no human soul); and Saint Cyril of Alexandria and the Council of Ephesus, c. 431, (against Nestorius who denied that Christ was but one Person and that His mother was thus truly the mother of God).
These "Christological heresies" generally came from exaggerations of some dogmatic truth at the expense of others. Around 450, Eutyches of Constantinople argued that the human nature of Christ was somehow "swallowed up" in the divine nature. He was condemned by the Council of Chalcedon and Pope Saint Leo the Great. A similar heresy proclaiming "monothelitism" -- that Christ had a human soul but only a divine will – was condemned by Pope Saint Martin in 649 at a Council held in the Lateran Basilica.
A most insidious heresy against the union of God and man originated before Christ in Manachieeism, the idea that a good god created the spiritual universe, and that an equally powerful but evil god made all material things. In the 13th century Catharism or Albigensianism not only rejected the hypostatic union, but held suicide in high honor.
The Church celebrates the hypostatic union in Its liturgy and private prayers. The Mass and related Eucharistic devotions are opportunities to worship our Lord, in both His humanity and divinity, under the appearances of bread and wine. As a re-presentation of the Sacrifice of the Cross, the Mass offers God a propitiatory sacrifice -– the only one adequate for mankind to atone for its sins against its infinite God -– adequate only because the offering is of the one Person who is both God and man. In Holy Communion we receive the Body and Blood of Christ -– humanity and divinity – and we worship Him as God-man in the tabernacle and at benediction and adoration of the Blessed Sacrament. "Blessed be Jesus Christ, true God and true man."
The Nicene Creed is recited frequently at Mass, and the Athanasian Creed occasionally in the Divine Office. The beginning of Saint John’s Gospel is read in all but a handful of Masses. The incarnation is the theme of the Masses of late Advent and the Christmas season, as well as on March 25th. The "overshadowing" of the Blessed Virgin is mentioned in the Preface of each Mass in honor of the Blessed Virgin and Saint Joseph, as well as in many of her Gospel readings. The incarnation and nativity are meditations of the joyful mysteries of the Rosary, recited at least twice a week by many Catholics – and the same meditations form the basis of the three-times-daily Angelus.