The Lord Jesus, on the night before he suffered on the cross, shared one last meal with his disciples. During this meal our Savior instituted the sacrament of his Body and Blood. He did this in order to perpetuate the sacrifice of the Cross throughout the ages and to entrust to the Church his Spouse a memorial of his death and resurrection. As the Gospel of Matthew tells us:
While they were eating, Jesus took bread, said the blessing, broke it, and giving it to his disciples said, "Take and eat; this is my body." Then he took a cup, gave thanks, and gave it to them, saying, "Drink from it, all of you, for this is my blood of the covenant, which will be shed on behalf of many for the forgiveness of sins." (Mt 26:26-28; cf. Mk 14:22-24, Lk 22:17-20, 1 Cor 11:23-25)
Jesus gives himself to us in the Eucharist as spiritual nourishment because he loves us. God's whole plan for our salvation is directed to our participation in the life of the Trinity, the communion of Father, Son, and Holy Spirit. Our sharing in this life begins with our Baptism, when by the power of the Holy Spirit we are joined to Christ, thus becoming adopted sons and daughters of the Father. It is strengthened and increased in Confirmation. It is nourished and deepened through our participation in the Eucharist. By eating the Body and drinking the Blood of Christ in the Eucharist we become united to the person of Christ through his humanity. "Whoever eats my flesh and drinks my blood remains in me and I in him" (Jn 6:56). In being united to the humanity of Christ we are at the same time united to his divinity. Our mortal and corruptible natures are transformed by being joined to the source of life. "Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me" (Jn 6:57).
By being united to Christ through the power of the Holy Spirit dwelling in us, we are drawn up into the eternal relationship of love among the Father, the Son, and the Holy Spirit. As Jesus is the eternal Son of God by nature, so we become sons and daughters of God by adoption through the sacrament of Baptism. Through the sacraments of Baptism and Confirmation (Chrismation), we are temples of the Holy Spirit, who dwells in us, and by his indwelling we are made holy by the gift of sanctifying grace. The ultimate promise of the Gospel is that we will share in the life of the Holy Trinity. The Fathers of the Church called this participation in the divine life "divinization" (theosis). In this we see that God does not merely send us good things from on high; instead, we are brought up into the inner life of God, the communion among the Father, the Son, and the Holy Spirit. In the celebration of the Eucharist (which means "thanksgiving") we give praise and glory to God for this sublime gift.