Sin, as action, means willingly and freely choosing to do something that we know is bad or wrong. Paradoxically, sin commonly involves choosing to do something that we somehow think is good. In his letter to the Romans, St Paul wrote, 'For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate… . For I have the desire to do what is right, but not the ability to carry it out. For I do not do the good I want, but the evil I do not want is what I keep on doing' (Romans 7:15-19). Like St Paul, we too are basically good, created in the image of a loving God who wants us to live wholeheartedly and who invites us to freely return that love in the way we relate to God and others. With the capacities of reason, free choice and love, we are capable of freely returning God's love for us. Yet we are also flawed. We experience our own faults and weaknesses as well as the faults and weaknesses of others. We can find ourselves knowing what is good and right yet struggling to do what is good and right. When we know what is right and good and nonetheless choose to do something other than that, then we sin.

Another way to think about sin is to think about it in terms of our relationship with God. God wants us to flourish, to live whole-heartedly. God has created the world in a way that makes it possible for us to flourish provided we choose to do the morally right thing in line with the way God has intended. Consequently, when we choose to do the morally wrong thing, we are not simply choosing something which is less good for our own flourishing and the flourishing of others, but we are choosing something which is against the good that God wants for us and for everybody else. Consequently, we are choosing against God. As Ronald Rolheiser (1999, p. 202) notes, 'All sin is irreverence.'

With freedom comes responsibility. We are responsible for our moral behaviour because we are made in God's image as rational beings, capable of knowing what the morally right and good thing to do is and as free beings, capable of choosing to do the morally right and good thing. These two capacities, to know and to choose, together form what is called conscience. Loosely translated, conscience means 'with knowledge'. In other words, when we make moral choices, we make them based on what we know about the goals we want to achieve, the ways or means to achieve them, the circumstances in which we need to achieve them and the consequences of both the means we choose and the outcomes we achieve. When we have weighed all these things, we make a judgment based on our knowledge of what the morally right thing to do is. We are then obliged to follow our conscience and do the morally right thing, taking responsibility for our decision.