The Eucharist is the heart and the summit of the Church's life, for in it Chirst associates His Church and all her members with His sacrifice of praise and thanksgiving offered once for all on the cross to His Father; by this sacrifice he pours out the graces of salvation on His Body which is the Church.
The Eucharistic celebration always includes: the proclamation of the Word of God; thanksgiving to God the Father for all His benefits, above all the gift of His Son; the consecration of bread and wine; and participation in the litergical banquet by receiving the Lord's Body and Blood. These elements constitute one single act of worship.
The Eucharist is the memorial of Christ's Passover, that is, of the work of salvation accomplished by the life, death, and resurrection of Christ, a work made present by the litergical action.
It is Christ himself, the eternal high priest of the New Covenent who, acting throught the ministry of the priest, offers the Eucharistic sacrifice. And it is the same Christ, really present under the species of bread and wine, who is the offering of the Eucharistic sacrifice.
Only validly ordained priests can preside at the Eucharist and consecrate the bread and the wine so that they may become the Body and Blood of the Lord.
By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about. Under the consecrated species of bread and wine Christ himself, living and glorious, is present in a true, real, and substantial manner: His Body and His Blood, with His soul and His divinity.
As sacrificed, the Eucharist is also offered in reparation for the sins of the living and the dead and to obtain spiritual or temporal benefits from God.
Anyone who desires to receive Christ in Eucharistic communion must be in the state of grace. Anyone aware of having sinned mortally must not receive communion without having received absolution in the sacrament of penance.
Communion with the Body and Blood of Christ increases the communicant's union with the Lord, forgives his venial sins, and preserves him from grave sins. Since receiving the sacrament strengthens the bonds of charity between the communicant and Christ, it also reinforces the unity of the Church as the Mystical Body of Christ.