MORTAL VS. 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 sinallows 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.
*One must not recieve Holy Communion when knowingly in a state of grave sin.* Doing so, is known and sacrilige. Mortal sin must be brought to the Sacrament of Confession before recieving the Eucharist.
Dr. Taylor Marshall writes about this here: http://taylormarshall.com/2016/05/receiving-sacraments-in-mortal-sin-what-happens.html
The spiritual effects of
the sacrament of Penance are:
-CCC 1496
1423 It is called the sacrament of conversion because it makes sacramentally present Jesus’ call to conversion, the first step in returning to the Father5 from whom one has strayed by sin. It is called the sacrament of Penance, since it consecrates the Christian sinner’s personal and ecclesial steps of conversion, penance, and satisfaction.
1424 It is called the sacrament of confession, since the disclosure or confession of sins to a priest is an essential element of this sacrament. In a profound sense it is also a “confession”—acknowledgment and praise—of the holiness of God and of his mercy toward sinful man.
It is called the sacrament of forgiveness, since by the priest’s sacramental absolution God grants the penitent “pardon and peace.”
It is called thesacrament of Reconciliation, because it imparts to the sinner the love of God who reconciles: “Be reconciled to God.” He who lives by God’s merciful love is ready to respond to the Lord’s call: “Go; first be reconciled to your brother.”
1440 Sin is before all else an offense against God, a rupture of communion with him. At the same time it damages communion with the Church. For this reason conversion entails both God’s forgiveness and reconciliation with the Church, which are expressed and accomplished liturgically by the sacrament of Penance and Reconciliation.
1441 Only God forgives sins. Since he is the Son of God, Jesus says of himself, “The Son of man has authority on earth to forgive sins” and exercises this divine power: “Your sins are forgiven.” Further, by virtue of his divine authority he gives this power to men to exercise in his name.
1442 Christ has willed that in her prayer and life and action his whole Church should be the sign and instrument of the forgiveness and reconciliation that he acquired for us at the price of his blood. But he entrusted the exercise of the power of absolution to the apostolic ministry which he charged with the “ministry of reconciliation.” The apostle is sent out “on behalf of Christ” with “God making his appeal” through him and pleading: “Be reconciled to God.”
RECONCILIATION WITH THE CHURCH CCC 1443-1445
1443 During his public life Jesus not only forgave sins, but also made plain the effect of this forgiveness: he reintegrated forgiven sinners into the community of the People of God from which sin had alienated or even excluded them. A remarkable sign of this is the fact that Jesus receives sinners at his table, a gesture that expresses in an astonishing way both God’s forgiveness and the return to the bosom of the People of God.
1444 In imparting to his apostles his own power to forgive sins the Lord also gives them the authority to reconcile sinners with the Church. This ecclesial dimension of their task is expressed most notably in Christ’s solemn words to Simon Peter: “I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.” “The office of binding and loosing which was given to Peter was also assigned to the college of the apostles united to its head."
1445 The words bind and loose mean: whomever you exclude from your communion, will be excluded from communion with God; whomever you receive anew into your communion, God will welcome back into his. Reconciliation with the Church is inseparable from reconciliation with God.
1450 “Penance requires . . . the sinner to endure all things willingly, be contrite of heart, confess with the lips, and practice complete humility and fruitful satisfaction.”
CONTRITION
1451 Among the penitent’s acts contrition occupies first place. Contrition is “sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again.”
1452 When it arises from a love by which God is loved above all else, contrition is called “perfect” (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible.
1453 The contrition called “imperfect” (or “attrition”) is also a gift of God, a prompting of the Holy Spirit. It is born of the consideration of sin’s ugliness or the fear of eternal damnation and the other penalties threatening the sinner (contrition of fear). Such a stirring of conscience can initiate an interior process which, under the prompting of grace, will be brought to completion by sacramental absolution. By itself however, imperfect contrition cannot obtain the forgiveness of grave sins, but it disposes one to obtain forgiveness in the sacrament of Penance.
1454 The reception of this sacrament ought to be prepared for by an examination of conscience made in the light of the Word of God. The passages best suited to this can be found in the Ten Commandments, the moral catechesis of the Gospels and the apostolic Letters, such as the Sermon on the Mount and the apostolic teachings.
THE CONFESSION OF SINS
1455 The confession (or disclosure) of sins, even from a simply human point of view, frees us and facilitates our reconciliation with others. Through such an admission man looks squarely at the sins he is guilty of, takes responsibility for them, and thereby opens himself again to God and to the communion of the Church in order to make a new future possible.
1456 Confession to a priest is an essential part of the sacrament of Penance: “All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are most secret and have been committed against the last two precepts of the Decalogue; for these sins sometimes wound the soul more grievously and are more dangerous than those which are committed openly.”
1457 According to the Church’s command, “after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year.” Anyone who is aware of having committed a mortal sin must not receive Holy Communion, even if he experiences deep contrition, without having first received sacramental absolution, unless he has a grave reason for receiving Communion and there is no possibility of going to confession. Children must go to the sacrament of Penance before receiving Holy Communion for the first time.
1458 Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church. Indeed the regular confession of our venial sins helps us form our conscience, fight against evil tendencies, let ourselves be healed by Christ and progress in the life of the Spirit. By receiving more frequently through this sacrament the gift of the Father’s mercy, we are spurred to be merciful as he is merciful.
1495 Only priests who have received the faculty of absolving from the authority of the Church can forgive sins in the name of Christ.