No, not just anyone can receive the Holy Communion at mass. There are certain things which must be done, certain practices which must be carried out by anyone who is willing to receive the Holy Eucharist and whosoever it is must be in a certain state of mind and heart.
Some guidelines were given for receiving the Holy Communion by the National Conference of Catholic Bishops in 1986 which stated, “Catholics fully participate in the celebration of the Eucharist when they receive the Holy Communion in fulfillment of Christ’s command to eat his body and drink his blood. In order to be disposed properly to receive Communion, communicants should not be conscious of grave sin, have fasted for an hour and seek to live in charity and love with their neighbours. Persons conscious of grave sin must first be reconciled with God and the Church through the sacrament of penance. A frequent reception of the sacrament of penance is encouraged for all.”
So we see that in order to participate fully in the Holy Mass, we should receive the Holy Communion. However, the criterium for receiving the Holy Communion is that you must be in a state of Grace (A state whereby no grave sin has been committed and sanctifying Grace is still present). A grave sin is also called a mortal sin. Sins like abortion, fornication, adultery, murder and every other sin strongly connected to the “seven deadlies” (Lust, gluttony, avarice, sloth, anger, envy and pride). For a sin to be identified as a mortal sin, three things must be involved: a serious matter, giving your consent to it, you had time to reflect on the gravity of the sin. When all three conditions are present, the sin is undoubtedly a mortal sin.
Any Catholic who commits any grave (mortal) sin must reconcile with God and the Church through the sacrament of penance (confession) before receiving the holy Eucharist. If he receives Communion without going for confession, he commits sacrilege which is an even bigger sin (1st Corinthians 11:27). This could put your soul to great trouble.
As explained in one of my previous articles, Is Going to Mass and Confessing Every Grievous Sin a Guarantee for Heaven?, confessing your sins frequently and doing your penance puts you in a better position to be in a state of Grace. The more you go for confession, the better for your soul. Confession should not be limited to only once or twice a year because we find ourselves falling to sin more often than that. Sin is sin. No sin is less than the other and every sin is worth being confessed.
If Popes go for confession weekly, how much more we? At least, going for confession once a month would be fairly good. As Christians we should always feel contrition for our sins and the desire to confess our sins always. One who does not feel bothered about not confessing his sins for a long time has probably lost his senses to sin which is a call for alarm. All these stated above apply only to Catholics.
For non-Catholics who attend mass, the bishops’ say, “We welcome to the celebration of the Eucharist, those Christians who are not fully united with us. It is a consequence of the sad divisions in Christianity that we cannot extend to them a general invitation to receive Communion. Catholics believe that the Eucharist is an action of the celebrating community, signifying a oneness in faith, life and worship of the community. Reception of the Eucharist by Christians not fully united with us would imply a oneness which does not yet exist and for which we must all pray”.
As seen clearly in this statement, Christians of other communities are welcome at any time to join in the celebration of the Holy Mass but cannot be offered the Holy Communion. However, there is an exception for some Christian communities like the Eastern Orthodox churches which share similar faith and doctrines as the Catholic Church. This is clearly seen in the code of Canon law: Canon 844, section 3, which says “Catholic ministers may licitly administer the sacrament of penance, Eucharist and anointing of the sick to members of the Oriental churches (The Eastern Orthodox churches) which do not have full Communion with the Catholic Church, if they ask on their own for the sacraments and are properly disposed. This holds also for members of all the churches which in the judgement of the Apostolic See, are in the same conditions as the Oriental churches as far as the sacraments are concerned.
So, members of the Eastern Orthodox churches (Oriental churches) can be allowed to receive Communion at a Catholic mass if they request for it. The Eastern Orthodox churches separated from the Catholic Church in 1054 but still share similar doctrines as ours which is why they can be allowed to receive the Holy Eucharist. The same applies to all the churches which share the same beliefs as Catholics regarding the Eucharist and the other sacraments.
This excludes Protestant churches because they do not share similar beliefs with the Catholic Church regarding the Eucharist and all the sacraments.