Dear Tony:

I am sorry that you are struggling to understand the concept of "mortal sin."   The concept of mortal sin comes from the First Letter of John, Chapter 5, Verses 16 and 17 in the New Testament:  "If anyone sees his brother sinning, if the sin is not deadly (mortal), he should pray to God and He will give him life.  This is only for those whose sin is not deadly.  There is such a thing as deadly sin (mortal) about which I do not say that you should pray.   All wrongdoing is sin, but there is sin that is not deadly (venial)."  In the early Church, especially when Christians were universally being persecuted and martyred by the Roman Empire during the first, second, and third centuries, any believer who in the face of persecution betrayed the faith committed mortal sin.  Mortal sin, deadly sin, was considered so destructive to that person's relationship with the Church and God that it would cut off that person from the life of the Church and from God Himself.  So, mortal sin was always considered a complete and destructive breaking of one's relationship with God and Church (the Body of Christ on earth, see I Corinthians, Chapter 12).  After the great persecution ended in the fourth century, the Church expanded the meaning of mortal sin to include sin as adultery, any serious sin that would disrupt the unity of the body of believers, and even not attending the Divine Liturgy (the Mass) at least once a week on Sundays, the Lord's Day.  But I am going to tell you something that is important and may even surprise you:  mortal sin in NOT defined by the Ten Commandments or any of the Church's regulations.  Mortal sin is defined by a person's heart, their motivation for their sinful action.  The catechism of the Church states that the requirements for a sin to be deadly, mortal, are met in these three points:

A mortal sin, as distinct from a venial sin, must meet all of the following conditions:

1. Its subject must be a grave (or serious) matter;
2. It must be committed with full knowledge, both of the sin and of the gravity of the offense (though nobody is deemed to be ignorant of the moral law, embedded into the consciences of every human being);
3. It must be committed with deliberate and complete consent, enough for it to have been a personal decision to commit the sin.

Mortal sin utterly severs the sinner's relationship with God and Church.  It can only be repaired by true repentance and the asking for forgiveness from God and Church (confession, the Sacrament of Penance).   Mortal sin is committed not so much in regards of breaking a commandment or regulation, but by the motivation of the heart.   There are people who commit mortal and destructive sin because they want to do it.   And they don't care about the consequences and the destructive power of mortal sin.   These people will have much to answer to God in this life and in the life to come.  

In regards to your question about whether those who committed the "mortal sin" of eating meat on Fridays in the past and other such concerns, God will not judge a person on whether he or she broke a regulation of the Church or one of the commandments as described in Exodus, Chapter 20.  God will judge the heart for it is the motivation of the heart that defines whether any wrongdoing is a mortal (deadly) or venial (not deadly) sin.  Jesus made this quite clear several times in the gospels and especially in the Gospel of Mark, Chapter 7, Verse 15: "Nothing that enters one from the outside can defile that person; but the things that come out from within are what defile."   So, if a person ate meat on Friday with the full intention of completely breaking their relationship with God and Church, then that person is in mortal sin and God will judge him accordingly.  If a person ate meat on Friday accidentally or even forgot it was Friday or did it without the intention of committing mortal sin or is unable to complete the requirement of abstinence, then there is no mortal sin present in this situation and therefore the person would not have completely broke their relationship with God and Church. Their salvation is assured because their heart was still with God and they did not break faith with the Lord. The Church puts great value in fasting and abstinence as a spiritual exercise.   All spiritual exercises can make a person stronger through personal spiritual discipline and more able to stay free from the destructive force of sin.  A strong spirit/soul can say NO to sin and YES to God's love!  Now, the requirements for fasting and abstinence are only required for our spiritual life during Lent though the Church encourages all believers to use spiritual disciplines at any time (Ash Wednesday and Good Fridays are days of REQUIRED fasting and abstinence and the Fridays are Lent are days of REQUIRED abstinence from meat).   Objectively, purposely not participating in this spiritual discipline during the season of Lent would be considered a mortal sin.  Infirmed people, people with chronic illness or disease, or people who are unable physically or mentally to fulfill the expectations of fasting and abstinence are ALWAYS exempt from these Lenten spiritual exercises.

In reference to Sunday Mass, that comes from the commandment to keep holy the Sabbath day.   Even though the Ten Commandments are part of the Old Law and have been abrogated in favor of the New Covenant of Jesus Christ, we still keep the commandments as a map for living our faith in Jesus Christ.  Sunday Mass is so essential to the unity of the Church.  All believers in Jesus Christ are asked to participate in listening to the Word of God and the sharing in Holy Communion (see Acts of the Apostles, Chapter 2, Verse 42:  "they devoted themselves to the teachings of the apostles and to the communal life, to the breaking of the bread and to the prayers.")    So, the early Church began to celebrate the Christian Sabbath on Sunday because that was the day Jesus Christ was raised from the dead.  So, we have this description of Sunday as the day that Christians would meet:  Acts of the Apostles, Chapter 20, Verse 7:   "On the first day of the week when we gathered to break bread...” And Saint Paul always met with the Christian community on Sundays as described in I Corinthians, Chapter 16, Verse 2:  "On the first day of the week, each of you should set aside."  Even the Book of Revelation was written on Sunday as seen in Revelation, Chapter 1, Verse 10:  "I was caught up in the Spirit on the Lord's Day (Sunday)."  Sunday gatherings at church IS ESSENTIAL to being a Christian, to being a Catholic.  So essential is Sunday Mass to one's identity as a Catholic that the Church ruled that not participating in the "teachings of the apostles and the breaking of the bread and the prayers" would constitute a "mortal sin," a deliberate breaking of relationship with the Church and Jesus Christ.  Objectively, the Church considers missing Sunday Mass a mortal sin.   There are exceptions, of course.  Those who cannot attend and participate at Mass on Sunday because of illness, disability, work schedule, distant travel, or even great distance from the nearest Catholic parish, do not incur "mortal sin."   For those who cannot attend Mass on Sunday, I usually suggest that they attend Mass during the week to fulfill the obligation of hearing the Word of God and receiving Holy Communion at least once a week.  For those who are ill or cannot leave their homes for Mass, please call your local parish and arrange having Holy Communion brought to you.   For those who are not limited by illness, disability or other serious reason, not attending Sunday Mass would be similar to a family member not going to the family's Thanksgiving meal because they don't want to have anything to do with the family.   The Mass is a Thanksgiving Meal (the meaning of the word, Eucharist).  And the family of believers come to this sacred meal in which Jesus Christ is the center.   Not to do so would be considered an insult to the body of believers and to the Lord.  That's why the Church has put such emphasis on the participation of every Catholic at Sunday Mass.  But like I said before, mortal sin is determined not by the Church but by the true motivation of the human heart.  I cannot judge the motivation of the heart but God can.   And on the day of our death, we will stand before God who will peer into the deep recesses of my heart and judge me accordingly.  So, as to your question, "would they have gone to hell?"  You are going to have to ask God that question.  He ALONE has the answer.  

Keep moving forward in your faith, searching always for God’s truth.