Fourthly, attention must be paid that the priest have, at the time of consecration, the intention that the real substance of the bread and the substance of wine be transubstantiated into the real body and blood of Christ through the operation of the Holy Spirit.
He makes this invocation when he confects this mystery by praying and saying: “Send your Holy Spirit upon us and upon these gifts here offered and make this bread the precious body of your Christ, and that which is in this chalice the precious blood of your Christ, changing them by your Holy Spirit.”
Transubstantiation occurs immediately with these words, and the bread is transubstantiated into the real body of Christ and the wine into the real blood of Christ, with the visible appearances alone remaining.
Orthodox Confession of Faith, Peter Mogila, Metropolitan of Kiev (1633-1647)