Traditional Roman Catholic Teaching on the Eucharist teaches that the Holy Eucharist completes Christian initiation. Those who have been raised to the dignity of the royal priesthood by Baptism and configured more deeply to Christ by Confirmation participate with the whole community in the Lord's own sacrifice by means of the of the Eucharist.
At the last supper, on the night He was betrayed, our Savior instituted the Eucharistic sacrifice of his Body and Blood. This He did in order to perpetuate the sacrifice of the cross throughout the ages until He should come again, and so to entrust to His beloved Spouse, the Church, a memorial of His death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a Paschal banquet 'in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is give to us.'
The Source and Summit
"The Eucharist is 'the source and summit of the Christian life.' The other sacraments, and indeed all ecclesiastical ministries and works of the apostolate, are bound up with the Eucharist and are oriented toward it. For in the blessed Eucharist is contained the whole spiritual good of the Church, namely Christ Himself.
The Eucharist is the efficacious sign and sublime cause of that communion in the divine life and that unity of the People of God by which the Church is kept in being. It is the culmination both of God's action sanctifying the world in Christ and of the worship men offer to Christ and through Him to the Father in the Holy Spirit."
- CCC # 1324 - 1325
What happens with the Eucharist is this: we believe that ther is a change of "substance" and there is no change in the "accident." What it looks like, acts like and tastes like is the same, but it is substantially changed to be something different.
The fact that there is no accidental change in the Eucharist is absolutely consistent with how God works. The change that takes place in the Eucharist takes place on the level of substance not on the level of accident. It is substantially changed. The change is absolute and real.
How does it happen?
At the Last Supper, Jesus took bread and wine and turned it into His Body and Blood. Jesus said the words,
"This is my Body and this is my Blood."
The only thing in all of creation that can choose not to obey God is humans. We have been given free will. Creation does not have a choice but to obey God. If Jesus orders a star to fall - it falls. If Jesus orders a storm to stop - it stops. When Jesus picked up the bread and said, "This is my Body," the bread did not disobey Him. When Jesus took the wind and said, "This is my Blood," the wine did not say, "I'm going to disobey you." When Jesus ordered the wine to become His Blood, it had to obey or He was not God. To deny that would be to deny the divinity of Jesus Christ. I say to you that when Jesus Christ says, "This is my Body and this is my Blood" - it happened!