See Catechism, paras 1749-56 on the nature of a moral act in relation to intention and circumstance, 1776-94 on conscience, 1846-96 on the nature of sin, and 599-618 on the Atonement. What follows is not a summary of the Church’s teaching which you will find there but a reflection upon it - a "thinking through" of the Church’s teaching, which may or may not make sense to others.

Sin is any action done deliberately against a greater good for the sake of a lesser. A sin is also (metaphorically) a kind of debt: something stolen from God, something that we take from his glory. (Thus the Lord’s Prayer asks God to forgive our "debts" or "trespasses" against him.) Whenever we do wilful damage to another person, or to ourselves, we are trying to keep for ourselves or to spoil something that is intended by God for another and a better purpose.

Sin always rebounds on the sinner. It is not possible for a sinner not to be punished. That is to say, every sin has an effect, not just on the world but on the person sinning. Those effects may be immediately visible (and intuitively it seems that obvious that the closer we are to God, the more immediate the effect), or they may be long delayed, in many cases only catching up with us after death. This is the order of justice: it is the way the world is made. If God had made it differently, the world would have made no moral sense. As it is, every action provokes a reaction, every cause has an effect, and sinners are automatically punished by their sin.

(The Church uses a more technical way of speaking about this. She talks about a "natural law" which reflects the "eternal law". These are not physical laws like those posited by science. Physical laws describe what happens when something happens in the physical order: a dropped ball rolls downhill, a split atom generates a certain amount of energy, and so on. The moral law describes what happens when a free action is performed by a moral being. The whole moral order is the providential arrangement of the world by God designed to lead free beings to a state of perfect fulfilment and happiness.)

We are creatures who live in time, but are destined to live in eternity. The consequences of sin are therefore twofold. The effects are felt both in time and in eternity. All sins cause suffering in time, but to the extent they constitute an actual rejection of friendship with God they also destroy life in the soul, which is eternal life in union with the Trinity. They block the flow of grace into the soul. Repentance and forgiveness can restore that friendship, and can unblock the springs of grace, but they do not by themselves prevent the other effects of sin, which continue to unfold in time. To minimize those effects we must do something to compensate for them. We must do something which has an equal and opposite effect to the sin itself. We must restore what was taken, from God and from our neighbour, or something like it.

That, again, is part of the order of justice, and it lies behind the ancient human instinct to make reparation and to offer sacrifices to the "gods" in order to restore a right relationship with heaven. However, such acts of reparation by themselves cannot restore the relationship with God, partly because our very capacity to act with pure motives has been impaired by the sin itself, and by other sins. By separating us from the infinity of God, any sin creates an infinite gap between us and him. Forgiveness by God can therefore only be an act of divine mercy, never something we can "earn" or "deserve" by what we do on our own to make up for our sins. The healing of our souls takes place by virtue of God’s acting in us. God’s assumption of human nature in the Son bridges that infinite gap between Creator and creature in order to restore man to a state of justice. When God the Son became man, he was able to pay back to his Father everything that we had taken from God. This he did by giving, as man, in place of what had been taken, something else of infinite value. That gift was nothing less than his own life, poured out for us on the Cross.

The great mystery here is the nature of this "giving", and the connection between Christ’s death and ourselves. Why was his death not just the ending of his life? How did it affect us? In what sense was it a "gift"?

To say it was a gift is to say that it was imparted to others. This imparting was accomplished by the Holy Spirit, the third person of the Holy Trinity. In something of the way that human love analogously unites two lovers in a single spirit, the Holy Spirit joins together those who love God. He constitutes the unity (communion) of the Church. Through the Holy Spirit, the very gift of God’s love incarnate in Christ is offered to all. Since that gift is received perfectly in Mary, it is with her that the communion of the Church begins. It is received by others to the extent that they are joined to her in the Spirit and become members of that communion

If all this seems a bit abstract, it is made concrete by the sacraments. There is nothing abstract about the Incarnation, the Crucifixion or the Resurrection. The sacraments extend the concreteness of the Incarnation through time and space. They are acts of Christ implanted in the Church by which we are physically connected with Christ – literally "incorporated". Christ has saved us. Now, by faith, charity and the sacraments, we can appropriate that salvation. If we welcome the Holy Spirit in baptism and confirmation, Christ will live in us and we in him. As the Catechism says, the divine person of the Son "surpasses and embraces all human persons", making possible his redemptive sacrifice for all (see para 616). This means that potentially our will and his will are united. Strengthened and healed by him from within, by the power of the Holy Spirit imparted through the sacraments, I will be able to love with a truly pure heart for the first time. I will be able to do good deeds with his grace. I will be able to do penance, and even the small actions prescribed by my confessor will have a great power to compensate for the effects of sin, because they are done with Christ living in me.