In his fourtieth Oration, while speaking to catechumens about to be baptized, Gregory the Theologian makes a very arresting claim: "I will be your Moses". To understand the context for this statement, it's helpful to understand some of the language that the Fathers used when speaking about God's grace effecting us, so perhaps it'd be best to start there.
We find many passages in the Fathers saying that God is like a divine artist and "draws" or "paints" onto us: onto our soul and our hearts. Many times, this drawing or painting on us is seen as happening through the divine mysteries or the divine virtues. Diadochos of Photiki, for instance, says:
"Divine grace confers on us two gifts through the baptism of regeneration, one being infinitely superior to the other. The first gift is given to us at once, when grace renews us in the actual waters of baptism and cleanses all the lineaments of our soul, that is, the image of God in us, by washing away every stain of sin. The second--our likeness to God--requires our cooperation. When the intellect beings to perceive the Holy Spirit with full consciousness, we should realise that grace is beginning to paint the divine likeness over the divine image in us.
Artists first draw the outline of a man in monochrome, and then add one color after another, until little by little they capture the likeness of the subject down to the smallest details. In the same way the grace of God starts by remaking the divine image in man into what it was when he was first created. But when it sees us longing with all our heart for the beauty of divine likeness and humbly standing naked in its atelier, then by making one virtue after another come into flower and exalting the beauty of the soul 'from glory to glory' (2 Cor. 3:18 ), it depicts the divine likeness on the soul.
Our power of perception shows us that we are being formed into the divine likeness, but the perfecting of this likeness we shall know only by the light of grace... Only when it has been made like God--in so far, of course, as this is possible-- does it bear the likeness of divine love as well. In portraiture, when the full range of colours is added to the outline, the painter captures the likeness of the subject, even down to the smile. Something similar happens to those who are being repainted by God's grace in the divine likeness: when the luminosity of love is added, then it is evident that the image has been fully transformed into the beauty of the likeness."
Love alone among the virtues can confer dispassion on the soul, for 'love is the fulfilling of the law' (Rom. 13:10) In this way, our inner man is renewed day by day through the experience of love, and in the perfection of love it finds its own fulfillment." - Diadochos of Photiki, On Spiritual Knowledge and Discrimination: One Hundred Texts, 89 (in: The Philokalia: The Complete Text, Volume One, [Faber and Faber, 1979], p. 288)
The discussion begins by talking about Baptism, and ends talking about love: both are divinely given methods for confering grace to us, and both can be used by God to make us beautiful and paint upon us the image He wants us to be. No one denies that it is God Who is the painter, but the Orthodox view of synergy is also very easily seen. In Saint Ambrose too we see this same concept of God painting on us and we participating synergetically:
"The soul is painted by God, that soul which has in itself the steadying grace of virtues and the splendor of piety. That soul is well painted, in which the likeness of the divine operation is reflected. The soul is well-painted, in which there is a splendor of glory and the image of the substance of the Father. In accord wtih this image, which shines forth, the picture is a priceless one. Adam, before his sin, was just such an image; but when he fell, he lost the image of the heavenly and took on the image of the terrestrial." - Ambrose, Hexameron, 6, 7, 42
"You are a portrait, O man, a portrait painted by your Lord God. Yours is a good artist and painter. Do not deface the good picture, which reflects not deceit, but truth, which expresses not guile but grace" - Ambrose, Hexameron, 6, 8, 46
Saint Ephraim the Syrian goes one step further, and us to paint virtue on our own hearts (though of course Saint Ephraim fully acknowledged that we couldn't do this in and of ourselves):
"Paint, O Youthfulness, thy victories on thy members, by which thou wilt become precious when thou growest old: paint on thy hands all charitable acts, with the visiting of the sick seal thy footsteps; paint on thy heart the image of thy Lord." - Saint Ephraim the Syrian, A Discourse on Virginity, 50 (cf Saint Ephraim the Syrian, The Pearl: Seven Hymns on the Faith, 5, 1)
When we come to Gregory the Theologian, we find slightly different language, but the same exactly concept these other Fathers affirmed. Instead of painting, Saint Gregory prefers to talk about inscribing or writing on "tablets of the heart" (cf Gregory the Theologian, Oration 43, 67). Certainly this writing is between God and the person being written on, but since the Priests have a role to play in providing the mysteries and helping us with the divie virtues, Saint Gregory sees the Priests as writing on people's hearts (through God's grace, of course). Sometimes, though, the tablets of the heart must be cleaned off first, as when they have sin or error inscribed on them:
"And he [ie. a Priest] must not only wipe out the traces of vice from his soul, but also inscribe better ones" - Gregory The Theologian, Oration 2, 14
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Gregory does not just see the Priest as someone who goes beyond only making sure people have access to the mysteries of God. Even apart from the sacramental role, the Priest has the duty to help people grow in other ways through the divine virtues and gifts of God's grace. The Priest also has the role of trying to heal a person and overcome their past errors in belief (the infamous "baggage" people bring with them when they become Orthodox):
"Accordingly, to impress the truth upon a soul when it is still fresh, like wax not yet subjected to the seal, is an easier task than inscribing pious doctrine on the top of inscriptions--I mean wrong doctrines and dogmas--with the result that the former are confused and thrown into disorder by the latter. It is better indeed to tread a road which is smooth and well trodden than one which is untrodden and rough, or to plough land which has often been cleft and broken up by the plough: but a soul to be written upon should be free from the inscription of harmful doctrines, or the deeply cut marks of vice: otherwise the pious inscriber would have a twofold task, the erasure of the former impressions and the substitution of others which are more excellent, and more worthy to abide." - Gregory the Theologian, Oration 2, 43
"If thy heart is written upon in some other way than as my teaching demands, come and have the writing changed; I am no unskilled caligrapher of these truths. I write that which is written upon my own heart; and I teach that which I have been taught, and have kept from the beginning up to these grey hairs. Mine is the risk; be mine also the reward of being the Director of your soul, and consecrating you by Baptism. But if you are already rightly disposed, and marked with the good inscription, see that you keep what is written, and remain unchanged in a changing time concerning an unchanging Thing. Follow Pilate's example in the better sense; you who are rightly written on, imitate him who wrote wrongfully. Say to those who would persuade you differently, what I have written, I have written. - Gregory the Theologian, Oration 40, 44
Gregory sees priests as correcting past errors and healing people. Not healing through "therapy" or "counselling," but through the holy mysteries and the holy virtues. The Priest can do this because of the grace of God working within the Church. Met. Hierotheos talks a good bit about such issues in his books (a number of which have thankfully been translated into English).
As Gregory says, the best type of person to be able to draw on are those who have not previously been drawn on. And this brings us to the original phrase quoted by Gregory: how he will be the catechumen's Moses:
"But not yet perhaps is there formed upon your soul any writing good or bad; and you want to be written upon today, and formed by us unto perfection. Let us go within the cloud. Give me the tables of your heart; I will be your Moses, though this be a bold thing to say; I will write on them with the finger of God a new Decalogue. I will write on them a shorter method of salvation. And if there be any heretical or unreasoning beast, let him remain below, or he will run the risk of being stoned by the Word of truth." - Gregory the Theologian, Oration 40, 45