The Church sets out specific guidelines regarding how a person should prepare to receive the Lord’s body and blood in Communion, About Catholics & First Holy Communion . To receive Communion worthily, you must be in a state of grace, have made a good confession since your last mortal sin, believe in transubstantiation, observe the Eucharistic fast and not be under an ecclesiastical censure such as excommunication.

A mortal sin is any sin whose matter is grave and which has been committed willfully and with knowledge of its seriousness. Out of habit and out of fear of what those around them will think if they do not receive Communion, some Catholics, in a state of mortal sin, choose to go forward and offend God rather than stay in the pew while others receive the Eucharist. You must have been to confession since your last mortal sin. The requirement for sacramental confession can be dispensed if four conditions are fulfilled:
* there must be a grave reason to receive Communion (for example, danger of death),
* it must be physically or morally impossible to go to confession first,
* the person must already be in a state of grace through perfect contrition and
* he must resolve to go to confession as soon as possible.

The guidelines for receiving Communion, which are issued by the U.S. bishops and published in many missalettes, explain, "We welcome our fellow Christians to this celebration of the Eucharist as our brothers and sisters. We pray that our common baptism and the action of the Holy Spirit in this Eucharist will draw us closer to one another and begin to dispel the sad divisions that separate us. We pray that these will lessen and finally disappear, in keeping with Christ’s prayer for us that they may all be one”.

"Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom Catholics are not yet fully united are ordinarily not admitted to Communion. Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law.

Scripture is clear that partaking of the Eucharist is among the highest signs of Christian unity. For this reason, it is normally impossible for non-Catholic Christians to receive Holy Communion, for to do so would be to proclaim a unity to exist that, regrettably, does not. Another reason that many non-Catholics may not ordinarily receive Communion is for their own protection, since many reject the doctrine of the Real Presence of Christ in the Eucharist. Scripture warns that it is very dangerous for one not believing in the Real Presence to receive Communion.

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