St. John Chrysostom (PG 59:261)
This blood is the salvation of our soul; it cleanses our souls, it beautifies our soul; ... it makes it shine even more than gold. Through the pouring out of this blood, it becomes possible to walk the path of heaven.
St. Augustine, Sermons, [227] A.D. 391-430:
That Bread which you see on the altar, having been sanctified by the word of God, is the Body of Christ. That chalice, or rather, what is in that chalice, having been sanctified by the word of God, is the Blood of Christ. Through that bread and wine the Lord Christ willed to commend His Body and Blood, which He poured out for us unto the forgiveness of sins. If you receive worthily, you are what you have received.
St. Cyril of Alexandria, Commentary on the Gospel of Matthew 26,27, 428 A.D.:
Christ said indicating (the bread and wine): 'This is My Body,' and `This is My Blood,' in order that you might not judge what you see to be a mere figure. The offerings, by the hidden power of God Almighty, are changed into Christ's Body and Blood, and by receiving these we come to share in the life-giving and sanctifying efficacy of Christ.
Is it then your understanding of what these Fathers are saying that the reception of the Eucharist is an instant theosis, though one that can be lost? I just read a collection of snipped comments of perhaps 20 saints and Fathers on this subject. There seems to be no doubt that what +John Chrysostomos, +Augustine and +Cyril represent the consensus patrum and yet in reading these 20 odd saints' comments, I get the feeling that id it were proposed to them that a single reception of communion confers theosis they would reject that proposal. I say this because they nearly all talk about the reception of the Eucharist as a part, a primary part to be sure, of living a life which leads to becoming Christlike. They do this by talking about the appropriate preparation for reception, how the Eucharist strangthens us, how it is a cure for a sick soul and body, how we become "partakers" in Christ (but not divinized as in complete theosis), those sorts of things.