What is sin ?
It's a big a topic for a website like this, but briefly, 'sin' is a religious word for doing wrong. Most religions have ethical codes or laws laid down by God or one of his messengers: for example, Judaism has the Torah built around the Ten Commandments and Islam has Sharia. Breaking these laws is an offence against God and so a religious person should devote their life to keeping them. Some religions have a twist on this, acknowledging that our human nature is such that we can never keep all these laws.
According to the Jewish teaching in the story of Adam and Eve, humans were made 'in the image and likeness of God' and placed in the pristine Garden of Eden. But because they disobeyed God's law, that likeness was damaged, and so they were banished from the garden to live the life that we associate with the human condition - a bewildering capacity for both good and bad deeds and thoughts, love and hate, hope and despair, faith and betrayal, aggression and reconciliation. This what is sometimes meant by the term 'original sin'.
Christianity inherits Adam and Eve and the Ten Commandments from Judaism, but puts a different spin on them. Jesus preached that the Ten Commandments, many of which are negatives ('Thou shalt not...'), can be turned round and summed up in two positives: 'Love God' and 'Love your neighbour (i.e. your fellow humans) as yourself', while St Paul argued that it is the Spirit, not the law, that gives life . Christianity portrays Jesus as the second or last Adam, another man made in the image of God who through his self-sacrificial love and forgiveness rescues humans from their failings and restores them to the likeness of God … if they acknowledge their need for it. Penthos is exactly that: an acknowledgment or recognition of one's failings and of the need for God's forgiveness and the joy of an authentic relationship with God that follows.