Turn away from sin and believe (be faithful to)  the Good News is one of the two sets of words that are to be said by a minister as he/she distributes ashes on Ash Wednesday (the other being  remember, man, you are dust and to dust you will return). Whereas both sets of words are equally meant to help us properly transition into the Lenten season by calling our attention to the fact that we should put our priorities in order, I always prefer the first set of words because of its emphasis on what Lent is all about:  LIFE/LIVING. This set of words is a reminder to us that the Lenten call of turning away from sin is (and should be) observed, not for its own sake, but rather for the good that it aims to preserve (which is  LIFE). 
When God created Adam and Eve, our first parents, God breathed upon them the breath of life and put them in the Garden of Eden whereupon God surrounded them with all that was necessary to sustain their lives. From the creatures of the earth that God put at Adam and Eve’s disposal, they were to get their food. God was also within their reach both as their ‘companion’ and friend (in other words, as someone with whom they were in relationship). And as long as Adam and Eve remained in the Garden of Eden, they were assured of life since this is what God had planned for them. However, the first couple sought to estrange themselves from God and from their fellow creatures (they ignored the plants and animals that God had permitted them to use as food and instead went for that which God had prohibited them to touch). By so doing, Adam and Eve interfered with the relationship into which God had inserted them and as a result found themselves thrown out of the garden of life. Instead of basking in the life-giving presence of God, Adam and Eve found themselves staring at death. They were no longer surrounded by the animals and plants which they could use for food, and God was now far from their reach. Finding themselves outside the warmth of relationship, it was the beginning of the end for them. To regain their former status as friends of God, they had to find their way back into a relationship with God. It is this that the Lenten theme of turning away from sin is all about. 
Lent calls upon us to turn away from that which contributed to Adam and Eve’s ouster from the garden of life (rebellion). We have to turn away from acts/behaviors that injure the relationship for which God created us. As a people created for relationship (interdependence), we have to desist from: looking for power in order to control or manipulate others (third temptation of Jesus); rejecting divine providence by seeking to provide for ourselves in the process of which we break the bonds of relationship (first temptation of Jesus); striving for independence from God and the society at large. We have to make amends with God and with our fellow creatures. This we do by believing the Good News which Jesus came proclaiming, and thereafter remaining faithful to it. 
In the Second Reading, St. Paul reminds us, albeit in a subtle manner, of the identity and mission of Jesus Christ as he refers to him as the new (a type of) Adam. Jesus, the new Adam, comes to repair the damage that resulted from the first Adam’s rebellion against God. Because of the first Adam’s rebellion, humanity became gripped by the power of sin leading to its revolt against the Creator. This revolt resulted (and continues to result) in both a damage and loss of the life-giving relationship that ought to exist between creatures and their Creator. And as long as this relationship remains strained, creatures will continue to suffer the effects of the first Adam’s acts of rebellion: death. Only in Jesus Christ has this harm been repaired. Unlike the first Adam who became the source of death for his descendants, Jesus, the second Adam, becomes a source of life for his brothers and sisters. 
In Jesus Christ, what had been lost when Adam and Eve were ousted from the Garden of Eden has been made available once again. In Jesus Christ, we can once again have access to that life for which we were created: a life of right relationship with God and amongst ourselves. Because of the new life given to us by Jesus, we now have the ability to reject any and all attempts to be like God by being aware of our special call as custodians of creation (cf. First Reading).