To understand Orthodox Christianity is to understand the Incarnation of Christ and the theology stemming from it.
God the Son became incarnate (took on flesh), by being born as a man from a Virgin. Thus, the invisible Word became visible, possessing the properties of our physical, human nature without it compromising His pre-eternal, divine one. Because of this the Orthodox Church holds to the unconditional truth that Christ has two equal and undivided natures, the human and the divine. In other words, Jesus Christ is fully God and fully man.
It is only through the incarnation that we know God. By His incarnation the Lord dwelt among us as one of us. It is in this way that He brought about our salvation. In that we are not just spirit but body, the Son of God deigned to come to us having a body like ours. Therefore, God ministers to us through both these aspects of our humanity. This means the all-wise Lord has allowed the invisible, spiritual things to be manifested to the whole person in a physically tangible and usable way.
This theme of the invisible reality being made manifest pervades all of Orthodoxy. The Incarnation refers specifically to the coming of Jesus Christ in the flesh. However, the principle of the incarnation, extending out from this like ripples in a pond, touches every other aspect of Orthodox Christian theology and sacramental life.
For example, it is because of the incarnation that we can venerate an otherwise invisible saint by means of his visible icon. Or that we can partake of what would be the inaccessible life-giving body and blood of Christ by means of the material bread and wine consecrated to be such. Likewise, that we can receive spiritual cleansing and re-birth from on high through the blessed water of Baptism. Even the Church building itself is affected, since it is understood to mystically contain Heaven within its walls.
How is this possible? Because when Christ entered the world, He sanctified it by His holiness as God. He sanctified the waters when he stepped into them at His baptism (Theophany). He sanctified the very air at His glorious Ascension. Most of all, He sanctified us, made in His image and likeness, by becoming one of us. Indeed, the whole world has been sanctified by His divinity having entered into it. This sanctifying of creation continues still because of his real presence in the Holy Mysteries of the Church of which we partake.
The Incarnation is not just one-time historical event. Rather, it is the Lord’s timeless legacy to us. By it the sinner has access to the treasure trove of God’s soul-saving spiritual gifts, made available even to our bodily senses in the mysteries of the Church, for the benefit of both body and soul. This is how the Lord continues to give of Himself. IT did not cease with the end of His earthly sojourn.
The Lord’s coming in the flesh for our salvation is yet another thread in the divine tapestry of His dispensation and love for us sinners. By it the Church has potency, and it is within the Church and her mysteries (sacraments) that we are grafted onto the Living Vine: our Lord and God and Savior, Jesus Christ. Thus, no one can say the Lord met us halfway. Clearly, He has met us all the way. “Who is so great a God as our God? Thou art the God who worketh wonders.”
Only the God of the Christian has, in His ineffable Love, humbly condescended to come down and unite His creation to Himself, saving us in such an intimate and personal way. In light of this, it is understandable why the teaching of the Incarnation endures as a cornerstone of Orthodox Christian theology now and always.