The most important doctrine (truth) of our Faith is the Blessed Eucharist, because it is the center around which the whole life of a good Catholic turns. Without participating at the Holy Sacrifice of the Alter, we are not Catholic. Without participating in this most holy, most necessary, most loving sacrifice of Our Lord Jesus Christ every Sunday, as God Commands, (ref. to the third Commandment) we are not Catholic, even though we call ourselves Catholic.

For Catholics, to miss Sunday Mass without serious reason, is to commit a mortal sin. To be worthy to receive the Blessed Eucharist in Holy Communion, the sinner must first receive absolution through Sacramental Confession. Not to do so is to commit a mortal sin of sacrilege!

Actions which prohibit persons from the reception of Holy Communion
In todays "Culture of death" the most prominent sins are, contraception, sterilization, abortion. Co-habitation (living together before marriage), adultery, pornography, active homosexuality, same sex marriage etc., voting for politicians who openly support abortion. Our Lady warned the children in Fatima that,
"More souls go to hell because of sins of the flesh than for any other reason"   "I have come to ask the faithful to amend their lives and ask pardon for their sins. They must cease offending God, who is already too much offended!" (Our Lady of Fatima, Pilgrim Virgin Committee).

Christ is the fulfillment of the Old Law. Everything in the Old Testament from the time of the disobedience of Adam and Eve, points to and prepares us for the coming of the Messiah - the Redeemer. The Law and the Prophets have their fulfillment in Him. He was shown forth in prophecy and in figure, This is particularly true of the Blessed Eucharist, by which He abides with His Church not only as God, but also as true Man; and in which those words of His are specially verified: "Behold I am with you all days, even to the consummation of the world." (Matt.28:20.) Therefore, in order to grasp fully the meaning of the words used by Our Lord when promising and instituting the Blessed Eucharist, we must know something about the history of the Chosen Jewish People of God and the figures that now give place to reality.

The Pascal Lamb
The whole Jewish race is descended from the twelve sons of Jacob. About seventeen centuries before the Christian era, one of them, Joseph, became Governor of Egypt; and during a great famine the rest of the brothers with Jacob their father came into the land of Egypt. The full history of these things you will find in the book of Genesis (37: to end.) The Hebrews in Egypt "multiplied and grew strong" (Exodus 1:20.) The Egyptians enslaved them, and devised ways to destroy them; until God raised up Moses to deliver His people from this slavery. The full story is found in the first chapters of the Book of Exodus.

Moses forced the Egyptians to let the Hebrews go free by bringing on Egypt ten plagues; the tenth plague was the most severe of all. The Hebrews were told by God to sacrifice a lamb without blemish and to sprinkle the door-posts of their houses with its blood. In every house, the door-posts of which had not been sprinkled with the blood of the lamb, the first born would die. So it came to pass: "And there arose a great cry in Egypt; for there was not a house wherein there lay not one dead." (Exodus 7:30.) Pharao, now terrified, let the Israelites go. "And the same day the Lord brought forth the children of Israel out of the land of Egypt by their companies." (Exodus 7:51.) In memory of this deliverance, God commanded the Israelites to celebrate a great feast, which they named the Pasch or Passover, and every year on this great feast they sacrificed a lamb called the Paschal Lamb, and ate of its flesh at the Paschal Feast.
This Paschal Lamb and the Paschal Feast are figures of Christ in the Blessed Eucharist.

The Manna
Moses had led the Israelites out of Egypt. God promised them a rich land which He would give them and in which they would live. But in the meantime they were wandering in the desert; and in their need they began to sigh again for the fresh meat of Egypt. In answer to the prayer of Moses God fed them miraculously from heaven. When they awoke one morning they found the ground about them covered with dew. It appeared small and white like frost. The Jews, seeing it, cried out "Manna," which means "What is this?"  Moses answered "This is the bread which the Lord has given you to eat." (Exodus 16:15.)
The Manna is a figure of the Blessed Eucharist.

Saint John tells us: "Now the Pasch, the festival day of the Jews was near at hand." Jesus lifted up His eyes and saw a very great multitude around Him. They had followed Him because of the miracles He had worked. Our Lord now worked another miracle and fed this vast multitude of five thousand men, besides women and children, with five barley loaves and two fishes; and after they had eaten, twelve baskets remained of the fragments left over. No wonder that the Jews now wished to take Him by force and make Him their King. But Jesus fled into the mountains alone.

The Jews now asked Him for a sign, reminding Him that Moses had given a sign by feeding their fathers with manna from Heaven. Our Lord told them. "I am the bread of life....if any man eat of this bread he shall live forever; and the bread that I will give is my flesh for the life of the world." (John 6:51.) Jesus spoke so clearly that there was only one way of understanding His words. He said He would give them His flesh to eat. The Jews were astonished and "they strove amongst themselves" saying: "How can this man give us His flesh to eat?" Jesus did not say to them: "You have misunderstood my meaning"; because they had not misunderstood His meaning. On the contrary, He told them that they must not only eat His flesh, but also drink His blood, if they desired everlasting life.
"Amen, Amen, I say unto you, unless you eat the flesh of the Son of Man, and drink His blood, you shall not have life in you." (John 5:54.)

Source: Rev. W. Frean, Commentary on the Catechism