Question: What did Christ mean when He said to St. Mary Magdalene, "...but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God" (John 20:17b)?
The simplest and most direct answer to this is given by Blessed Theophylact: "God is indeed our Father, but only by grace. He is Christ's Father by nature. On the other hand, the Father is our God by nature; He is Christ's God in respect to his human nature alone. Only after assuming human nature can Christ say the the Father is His God." (The Explanation of the Holy Gospel According to John. Fr. Christopher Stade, Trans. (House Springs, MO: Chrysostom Press, 2007), p. 298).
Here is what some other Fathers have to say to expound on this question further:
St. Cyril of Alexandria wrote: "I will explain, then, as far as I am able: In the first place, then, though we are servants by rank and nature (for creatures are subject to their Creator), He calls us His brethren, and designates God the common Father of Himself and us; and, making humanity His own, by taking our likeness upon Him, He calls our God His God, though He is His Son by Nature; that, as we mount up to His exceeding great dignity of station by likeness to Him (for it is not because we are by nature sons of God that we are so called, for He cries in our hearts by His own Spirit, Abba, Father), so also He, since He took our form -- for He became Man, according to the Scriptures -- might have God for His God, though He was truly God by Nature, and proceeded from Him. Be not, therefore, offended, though you hear Him calling God His God, but rather contemplate His words in a teachable spirit, and attentively consider their true meaning. For He says that God is both His Father and our God; and both sayings are true. For, in very truth, the God of the universe is Christ's Father, but not ours by nature; but rather our God as our Creator and Sovereign Lord. But the Son, as it were, blending Himself with us, vouchsafes to our nature the dignity that is in a special and peculiar sense His own, calling Him That begat Him the common Father of us all; while, on the other hand, He receives into Himself, by taking upon Him our likeness, that which belonged to our nature. For He calls His Father His God, being unwilling, through His inherent love and mercy toward mankind, to dishonour our likeness that He had taken upon Himself. If, then, you choose in ignorance to cavil at this saying, and it seem intolerable to you that the Lord should say that God the Father was His God, you will then, in your perversity, be bringing a charge against the scheme for your own redemption; and when you ought to be offering up thanksgiving you will be dishonouring your Benefactor, and be foolishly objecting to the manner in which He manifested His love towards you. For if He humbled Himself, despising shame, and became a Man for your sake, on your head is the charge of humiliation, and to Him Who chose to undergo this for your sake, exceeding great is the honour due. And I am amazed that you have ears merely for the eclipse of glory (for He humbled Himself for our sake), and consider not its restoration, and, regarding only the degradation, reflect not upon the exaltation. For how was He humiliated, if you do not regard Him as perfect, as being God? And in what sense was He degraded, if you do not take into account the lofty attributes of His ineffable Nature? Therefore, when He was perfect and all-sufficient as God, He humbled Himself for your sake, transforming Himself to your likeness; and though He was high exalted as the Son of God, and of the very Essence of the Father, He degraded Himself, being mulcted of the attributes of Divine glory, so far as His Nature admitted. As therefore, now, He is at the same time God and Man, being high exalted because of His parentage (for He is God of God and truly Begotten of His Father), and also made lowly for our sake (for He became Man for us); be of a tranquil mind when you hear Him saying: I ascend unto My Father and your Father, and My God and your God. For it was very meet and right that, as being by Nature God and Son of God, He should call Him That begat Him His Father; and that, as being Man, even as we are men. He should call God His God" (Homilies on the Gospel of John, Book XII, chapter 1).
St. Augustine said: "He saith not, Our Father: in one sense, therefore, is He mine, in another sense, yours; by nature mine, by grace yours. “And my God, and your God.” Nor did He say here, Our God: here, therefore, also is He in one sense mine, in another sense yours: my God; under whom I also am as man; your God, between whom and you I am mediator" (Homilies on the Gospel of John, Tractate 121:3).
St. Cyril of Jerusalem said: "But lest any one from simplicity or perverse ingenuity should suppose that Christ is but equal in honour to righteous men, from His saying, I ascend to My Father, and your Father, it is well to make this distinction beforehand, that the name of the Father is one, but the power of His operation manifold. And Christ Himself knowing this has spoken unerringly, I go to My Father, and your Father: not saying ‘to our Father,’ but distinguishing, and saying first what was proper to Himself, to My Father, which was by nature; then adding, and your Father, which was by adoption. For however high the privilege we have received of saying in our prayers, Our Father, which art in heaven, yet the gift is of loving-kindness. For we call Him Father, not as having been by nature begotten of Our Father which is in heaven; but having been transferred from servitude to sonship by the grace of the Father, through the Son and Holy Spirit, we are permitted so to speak by ineffable loving-kindness (Catechetical Lectures 7:7).
"But the Father having begotten the Son, remained the Father and is not changed. He begat Wisdom, yet lost not wisdom Himself; and begat Power, yet became not weak: He begat God, but lost not His own Godhead: and neither did He lose anything Himself by diminution or change; nor has He who was begotten any thing wanting. Perfect is He who begat, Perfect that which was begotten: God was He who begat, God He who was begotten; God of all Himself, yet entitling the Father His own God. For He is not ashamed to say, I ascend unto My Father and your Father, and to My God and your God. But lest thou shouldest think that He is in a like sense Father of the Son and of the creatures, Christ drew a distinction in what follows. For He said not, “I ascend to our Father,” lest the creatures should be made fellows of the Only-begotten; but He said, My Father and your Father; in one way Mine, by nature; in another yours, by adoption. And again, to my God and your God, in one way Mine, as His true and Only-begotten Son, and in another way yours, as His workmanship. The Son of God then is Very God, ineffably begotten before all ages (for I say the same things often to you, that it may be graven upon your mind). This also believe, that God has a Son: but about the manner be not curious, for by searching thou wilt not find. Exalt not thyself, lest thou fall: think upon those things only which have been commanded thee. Tell me first what He is who begat, and then learn that which He begat; but if thou canst not conceive the nature of Him who hath begotten, search not curiously into the manner of that which is begotten" (Catechetical Lectures 11:18-19).
We know that this text is not saying that Christ is not God, because at the very beginning of the Gospel of John, we are told that not only was the Word (Christ) with God, but that the Word was God (John 1:1). And later in the same chapter as the verse in question, St. Thomas, upon understanding that Christ was truly risen, addressed Christ, saying "My Lord and my God!" (John 20:28) -- and Christ did not rebuke or correct him, but rather confirmed his statement by responding "Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed" (John 20:29).