On Holy and Great Thursday we commemorate the Mystical Supper of Christ with His apostles. This is the "night he was betrayed, or rather, when he surrendered himself for the life of the world." On that evening Christ, mystically gave His body and blood to the disciples. He showed Himself to be the Lamb of God whose blood would ransom mankind from sin.On the night he was betrayed, or rather, when he surrendered himself for the life of the world, he took bread into his holy and all-pure and immaculate hands, gave thanks and blessed, sanctified, broke., and gave it to his holy disciples and apostles, saying: Take, eat; this is my body which is broken for you for the remission of sins. Likewise, he took the chalice after supper, saying: Drink of this all of you; this is my blood of the new covenant which is shed for you and for many for the remission of sins.The Divine Liturgy
It is proper and just to sing hymns to You, to bless You, to praise You, to thank You, to worship You in every place of Your Kingdom; for You are God ineffable, inconceivable, invisible, incomprehensible, ever-existing, yet ever the same, You, and Your only-begotten Son, and Your Holy Spirit; You brought us forth from nonexistence into being, and raised us up again when we had fallen, and left nothing undone, until You brought us to heaven and bestowed upon us Your future Kingdom. For all this we give thanks to You, and to Your only-begotten Son, and to Your Holy Spirit, for all that we know and that we do not know, the manifest and hidden benefits bestowed upon us. We thank You also for this Liturgy, which You have willed to accept from our hands... (Anaphora, before the 'Holy, holy, holy')
"The bread and also the wine mixed with water, through the epiclesis and coming of the Holy Spirit, are supernaturally changed into the Body and Blood of Christ.'' (Saint John Damascene).