Three things are required for a sin to be mortal: serious matter, full consent, and full knowledge (cf. CCC 1858-1859).
Serious matter means what has been done is just that—serious. It’s something with real consequences; it matters in my life with God and the Church. Our Church helps us by pointing out certain activities that are serious matters—usually significant violations of the 10 Commandments.
Full consent, the second requirement, means that we freely chose to do it. Doing anything with full freedom is rare because we are weak human beings who often think we are more free than we really are. Things like fear, peer pressure, laziness, addiction, ignorance, pride, and immaturity, often interfere with being truly free.
The same goes with full knowledge. Here we must ask: Did I truly understand and appreciate the ramifications and seriousness of what I did? Did I know it was wrong and fully grasp its sinfulness and its consequences for my life with God, self, and others?
Now here’s the most important thing to remember: To the degree that I was free or had knowledge of what I was doing, I am responsible for sinning. So I may have done something seriously wrong, but it may not be a serious or mortal sin. It is sinful and should be rooted out of our lives, but may not be mortally so.
Our loving God understands our human weakness. That is one reason he became human. He is calling us to full life with his Church and with himself.
Why can’t I confess my sins directly to God? Why involve a priest?
The Book of Genesis relates that God shared authority with human beings by having them rule over the animals. The Book of Exodus tells of God working through the ministry of Moses to liberate the people. The prophets were men who mediated between God and human beings. Jesus sent his disciples out to share in his ministry of healing and preaching.
God could obviously have done a better job than Adam and Eve, Moses, the prophets, the disciples, or any one of us. But love allows God to share with humans—even knowing that we will mess up. God chose to make certain people mediators of his love and grace for the good of the entire community so that humans would have a share in his ministry.
In each of the sacraments, God’s grace—for it is really his grace that is at the heart of every sacrament—comes to us mediated through people and things. That’s just the way God set it up because of his love for us.
So, too, in the Sacrament of Reconciliation. God forgives through the ministry of a priest. And this gives the sacrament a very warm and human aspect as we hear the words of absolution pronounced officially and authoritatively by a priest who not only represents Christ, but the whole Church. We are human beings with senses and emotions. The mediation of God’s graces speaks to our need to hear and sense his words of forgiveness.
How important is this sacrament in my daily faith journey?
Since we are required to celebrate the Sacrament of Reconciliation only when we are conscious of having committed a mortal sin (cf. CCC, 1457), the sacrament could, theoretically, play a very minimal role in our spiritual life. But that would be very minimalistic to say the least.
The Catechism of the Catholic Church strongly encourages more frequent celebration of the sacrament, stating that even confession of our less serious sins leads to positive effects. It lists the following: confession helps us form our consciences, helps us resist temptation, allows us to experience the healing touch of Christ, and helps us progress in the life of the Spirit (cf. CCC, 1458).
Frequent use of the sacrament also helps us stay attuned to our spiritual lives. The more frequently we become aware of the condition of our life with God, self, and others, the better grip we have on both the progress and lack of progress in our spiritual life. It offers a good barometer of how we are doing.
Making the Sacrament of Reconciliation a vibrant part of one’s spiritual life—especially if it hasn’t been such in quite a while—basically just requires the decision to begin again. Most priests would be willing to help someone back into the swing of things, knowing that the first time back could be a bit awkward.