Question: Is someone who was baptized into the Catholic Faith, but seldom if ever goes to Mass, still considered a Catholic?
Q: How can that be?
A: It can be because the "mark" of baptism is not something one can wash away. Denial of the mark does not make it go away. Once validly baptized - you cannot remove baptism.
Q: But if someone is baptized and never goes to a Catholic church anymore, in what way are they still Catholic?
A: They are Catholic, but not a "faithful Catholic." A faithful Catholic adheres to the decrees of the Church, one of which is that which is commonly called "the Sunday obligation." This is summarized in the Catechism of the Catholic Church in "the first precept:"
CCC 2042 The first precept ("You shall attend Mass on Sundays and holy days of obligation and rest from servile labor") requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts honoring the Mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days. The second precept ("You shall confess your sins at least once a year") ensures preparation for the Eucharist by the reception of the sacrament of reconciliation, which continues Baptism's work of conversion and forgiveness.
The third precept ("You shall receive the sacrament of the Eucharist at least during the Easter season") guarantees as a minimum the reception of the Lord's Body and Blood in connection with the Paschal feasts, the origin and center of the Christian liturgy.
Catechism of the Catholic Church 2042
Willful violation of any of these precepts would constitute a "mortal sin" - one which separates the Christian from Sanctifying Grace and therefore necessitating a return to the Sacrament of Reconciliation (Confession) prior to receiving the Eucharist again.
Q: So, should a Catholic who has missed Mass refrain from going to Mass until they have made it back to Confession?
A: No! Why compound the sin?! The requirement of "the first precept" is not necessarily to receive the Eucharist every Sunday - but to gather at the participation of the Eucharist. The "Sunday obligation" therefore is not fulfilled by receiving the Eucharist - but by BEING THERE. Reception of the Eucharist is only "required" at least once during the Easter season (which would be from the beginning of Lent until Pentecost Sunday). Now of course, if you are "there" to receive the Eucharist, you've already fulfilled the Sunday obligation. The reception of the Eucharist then is a renewal of Actual Grace for the Christian.
Q: Does a Catholic "keep holy" the Day of the Lord (Sunday) in attending a non-Catholic service?
A: For the most part, no. A Catholic could fulfill the Sunday obligation at an Orthodox church, but not at any Protestant church.
Q: Why does a Catholic not fulfill the first precept at a Protestant church?
A: Because it is not a valid celebration of the Eucharist. The Eucharist is key to and central to the Catholic Faith. While Protestants share in some measure of the truth, they do not share at all in the Eucharist and the precept specifically states that we are to participate in (not necessarily receive) the Eucharist. Therefore it is impossible to fulfill the "Sunday obligation" at a Protestant gathering.
Q: What should a Catholic do who has not been to Mass for weeks, months or even years?
A: Well, the first thing they should do is break the cycle of not going to Mass! GO! Get there THIS Sunday if at all possible. The next step is to reconcile the sin of the absences, and you should not wait long there either, for you stand in a state separated from Sanctifying Grace.
Q: But if I've been gone for so long, and willfully refused to go to Mass - how can God forgive me?
A: God can and will forgive you! Do not fear going back to the confessional! Scripture tells us of the joy and celebration, even in Heaven, when a sinner repents and reconciles him/herself.
Q: But why must I go to a priest for confession? Can't I just express my sorrow directly to God and start going to Mass and receiving the Eucharist again?
A: Well, primarily the reason you cannot just go and take care of this yourself is because God has not empowered you with the forgiveness of sins. You must go to one whom He has so empowered. Part two of your question, you cannot receive the Eucharist unworthily, for that would bring upon yet another layer of sin.
Q: So who has God empowered to forgive sins?
A: This is spelled out in Scripture, John 20:23 to be specific:
Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained.Now Jesus was speaking specifically to His Disciples, for only they were present at the time (see v. 19). So authority to forgive sins was given to them, the Disciples - our first bishops, thus the faculties to forgive sins belongs with the bishop - and to those whom the bishop has allowed to act in his stead (the priests of his jurisdiction). Therefore sins they do not forgive - are not forgiven. One can rationalize this all they want, but ultimately they cannot get around the fact that specific men were given this authority and outside these men, no one else has been given this authority.
Q: But I'm too embarrassed to go to confession and talk to a priest not only about missing Mass, but I've got some other sins I'd rather not talk about. Why do I need to go to the priest?
A: Well, that answer has already been given, just previously, but fear not! The priest hearing your confession has likely heard anything you have to confess numerous times already, you're not going to surprise him with some new sin... in fact there are no "new sins" - sin is just sin. The priest is bound by the secrecy of the confessional - that means he cannot talk about your confession to anyone. What you confess is basically between you and God through one of God's duly ordained representatives. Again, the fact that you have come back to the Church is cause for rejoicing and celebration! Come home! Just as the Prodigal Son returned, even after wasting away all his inheritance, the father welcomed him home with open arms, and had a huge celebration over the fact that his son, who was dead, has returned to him.
So, your primary responsibility is to get back to the Mass. No more excuses, just get back to Mass. Next, get to Confession. If you wish, you could arrange to visit with your local priest and discuss this with him prior to going to Confession. The most important thing is to stop putting it off. Come home.