I was told that receiving the sacrament of the Eucharist while in a state of grave sin is a grave sin in itself. I’m constantly worried about whether I am allowed to receive Communion at Mass, and I do make sure I go to confession monthly (or more often). My question is this: How 
grave must a sin be to have to abstain from the Eucharist? A group of us have discussed this and we got to the idea that you can receive communion if you are truly sorry for your sins, even if you haven’t yet gone to reconciliation for it, and it’s up to ourselves to judge if we’re worthy of receiving or not. Is that true?
To keep us from receiving Holy Communion, a sin has to be what we call a “mortal sin.” There are three conditions for a sin to be mortal: It has to have been committed with full knowledge that the action was gravely evil (you can’t commit a mortal sin “by mistake”); it has to be committed with full consent (I didn’t do it because I was afraid or pressured); and it has to involve what is called “grave matter,” which means the action itself has to be serious. Stealing a piece of bubble gum would not be a mortal sin (it would be a venial sin), but stealing a diamond necklace worth half a million dollars would be a mortal sin. Sometimes it’s hard to know if all three conditions have been met. In those cases, the best thing is to go to confession and ask the priest to help you discern, or ask someone who you know and trust and who has good wisdom on these kinds of things. Usually, though, if there is doubt in your mind, it is probably not a mortal sin. But we have to be careful here – it is our responsibility as mature Christians to inform ourselves about what constitutes “grave matter” and what doesn’t. For instance, a lot of people don’t realize that missing Sunday Mass is grave matter. A lot of people don’t think that getting drunk is grave matter either, but it is.
Understanding “Grave Matter”
The basic list of actions that constitute “grave matter” is the Ten Commandments. But it would be impossible to make a list of all permutations of actions in those ten categories. Nevertheless, we can reflect on the underlying reason why something would be considered grave. Whenever we choose something radically opposed to God’s goodness – like murder, fornication, or refusing to praise and thank God by attending Sunday Mass – we are, basically, rejecting God’s friendship. The object of our choice (the “matter” of the sin) is in direct and full opposition to the very heart of God. By choosing it, we are saying to God that we can live without him, that we don’t want him around. In this way, we destroy the theological virtue of charity in our soul. That constitutes a mortal sin, which God will readily forgive if we sincerely repent and go to him in the sacrament of confession. Whenever we give in to temptations that are opposed to God’s goodness in less radical ways – like sleeping in a little longer than necessary, eating a little more than is necessary, stealing small office supplies from an employer for personal use – we aren’t outright severing our friendship with God, but we’re distancing ourselves from him. These actions would not constitute grave matter. In these cases, we’re making small concessions to selfishness that close off certain sectors of our heart from his love and thereby weaken the theological virtue of charity in our soul. That constitutes a venial sin, which God will readily forgive if we sincerely repent even if we don’t go to confession, but which, if left unrepented, could easily snowball into the outright rebellion of mortal sin.
Sins vs Mistakes
Sometimes we have difficulty distinguishing between sins and simple mistakes. If I sincerely forget to send my mom a mother’s day card, I may have strong feelings of regret, but I shouldn’t feel morally guilty about it, I shouldn’t feel remorse. If I do, it’s a sign that my conscience is overactive, or scrupulous. If, on the other hand, I purposely avoid calling my mom on her birthday because I’m nursing resentment about something she said five years ago, then I ought to feel guilty; Christians honor their parents, they don’t hold grudges against them.
Why Communion and Mortal Sin Don’t Go Together
The reason the Church forbids us from receiving Holy Communion when we are in a state of mortal sin is precisely because mortal sin breaks our friendship with Christ. It’s like a divorce – we rebel against him by choosing to do something that pains him deeply; we walk out on him. But Holy Communion is a sacrament of intimate union and love. And so, before we can come back to Communion, we need to be reconciled with the Lord – we need to repent, confess, and receive his forgiveness. This is the point of the sacrament of reconciliation, or confession. (If there is some good reason why I cannot make a good confession before receiving Holy Communion, I can still receive Holy Communion if I make a perfect act of contrition, and if I have the intention of going to confession as soon as I can.) When we commit a venial sin, it’s like getting into an argument with a friend – we offend God, but it is not a total break with him. And so, Holy Communion is actually one of the ways that the Church recommends we use in order to have our venial sins forgiven. In all cases, though, as soon as we realize we have sinned, we should immediately pray to God to forgive us and ask him for help to avoid sinning in the future. God’s mercy is always available – even if we can’t make it to confession for a good reason. But if we can make it to confession, then we should (every confession is like a power-wash of grace for the soul, and it gives great glory to God, and it enhances our spiritual maturity immeasurably). You may be interested in reading what the Catechism has to say about mortal and venial sin, and grave matter. We reproduce it below for you convenience:
IV. THE GRAVITY OF SIN: MORTAL AND VENIAL SIN
1854 Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture, became part of the tradition of the Church. It is corroborated by human experience.
1855 Mortal sin destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.Venial sin allows charity to subsist, even though it offends and wounds it.
1856 Mortal sin, by attacking the vital principle within us – that is, charity – necessitates a new initiative of God’s mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation:When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object . . . whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery. . . . But when the sinner’s will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial.
1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: “Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother.” The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God’s forgiveness, it causes exclusion from Christ’s kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.
1862 One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
1863 Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul’s progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not break the covenant with God. With God’s grace it is humanly reparable. “Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness.” While he is in the flesh, man cannot help but have at least some light sins. But do not despise these sins which we call “light”: if you take them for light when you weigh them, tremble when you count them. A number of light objects makes a great mass; a number of drops fills a river; a number of grains makes a heap. What then is our hope? Above all, confession.
1864 “Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven.” There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss.
