The Mass is celebrated by Priest and People together. (Before the Second Vatican Council 1965 there was a widespread misconception that the Priest celebrated the Mass and the People were mere spectators.)
You may need to check the What? page for terminology before these next two paragraphs.
All people of any denomination, any faith, or none, are welcome to join the Catholic Community in celebrating the Mass. However, when it comes to Holy Communion in the Mass, ie receiving the Body and Blood of Christ in Holy Communion, this regrettably has to be reserved for practising Catholics. Holy Communion is the most sacred moment in the Mass, but that’s not real reason for this reservation. Holy Communion is the expression of full union with the Catholic Church and consummation of full union with the Catholic Church, so we have no option but to reserve this for those who are already fully practising members of the Catholic faith. However, we want all people to be as closely united in our celebration of the Mass as possible, so when it comes to that part of the Mass called Holy Communion, we invite all people of whatever denomination or faith or none to come forward and receive a blessing from the priest. The universally recognised signal for this is to cross your arms across your chest when you come to the priest, and he will give you a blessing.
Catholics who have not yet taken a preparation course for Holy Communion, this applies usually to children below the age of 7/8 (Year 3 at school) but could apply to people of any age, must undergo a preparation course given by the parish or in the parish school. At the end of this course they receive the Body and Blood of Christ in the Mass for the first time, after which it becomes a regular part of their sacramental life. This is sometimes referred to as “Making their First Holy Communion”. This constitutes full initiation into the Sacrament of the Eucharist and full participation in the Mass. See Eucharist page under the Sacraments tab for more information about our preparation course.