Receiving Holy Communion, while in a state of mortal sin, is a sin against the Lord

In the Holy Eucharist, Jesus is giving Himself to us in the form of bread and wine. The Lord is truly present in that tiny white host - Body, Blood and Divinity. Hence, it is important that when we receive Him in Holy Communion, we are under the state of grace. In other words, we are not in the state of mortal sin, or, if we currently are, we must confess our sins first and receive absolution in the Sacrament of Reconciliation. 

The Holy Scriptures and the Catechism of the Catholic Church preaches that whoever receives the Lord unworthily is guilty of profaning the Body and Blood of the Lord.

1. The Apostle Paul warns us “Therefore whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord.” (cf. 1 Corinthians 11:27) 
2. The Catechism also warns us “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.”  -Catechism of the Catholic Church 1457 

The Name of the Lord is HOLY. Be true to your promises made in His Name.

The Name of the Lord is Holy. It is a grave sin if we blaspheme the Holy Name of God. It is blasphemy if we broke our promises made in His Name. The Catechism of the Catholic Church (2147 and 2148) teaches us:

“Promises made to others in God's name engage the divine honor, fidelity, truthfulness, and authority. They must be respected in justice. To be unfaithful to them is to misuse God's name and in some way to make God out to be a liar.

Blasphemy is directly opposed to the second commandment. It consists in uttering against God - inwardly or outwardly - words of hatred, reproach, or defiance; in speaking ill of God; in failing in respect toward him in one's speech; in misusing God's name. St. James condemns those "who blaspheme that honorable name [of Jesus] by which you are called."  The prohibition of blasphemy extends to language against Christ's Church, the saints, and sacred things. It is also blasphemous to make use of God's name to cover up criminal practices, to reduce peoples to servitude, to torture persons or put them to death. the misuse of God's name to commit a crime can provoke others to repudiate religion. Blasphemy is contrary to the respect due God and his holy name. It is in itself a grave sin.”

God wants us to keep the sabbath holy. Skipping Sunday Mass, with no serious reason, is a sin.

God wants us to keep the sabbath holy. Skipping Sunday Mass, with no valid reason, is a sin. 

THE SUNDAY EUCHARIST ACCORDING TO THE CATECHISM OF THE CATHOLIC CHURCH (2177 and 2181)

"The Sunday celebration of the Lord's Day and his Eucharist is at the heart of the Church's life. "Sunday is the day on which the paschal mystery is celebrated in light of the apostolic tradition and is to be observed as the foremost holy day of obligation in the universal Church."

"The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin."

Since Sunday is a holy day of obligation, failure to attend and participate is a sin. However, there are circumstances where attendance to Holy Mass is not possible. Here, the catechism (2183) provides us a remedy: 

"If because of lack of a sacred minister or for other grave cause participation in the celebration of the Eucharist is impossible, it is specially recommended that the faithful take part in the Liturgy of the Word if it is celebrated in the parish church or in another sacred place according to the prescriptions of the diocesan bishop, or engage in prayer for an appropriate amount of time personally or in a family or, as occasion offers, in groups of families."