Christianity is at heart about how we relate to God. One of our key needs is to be justified before God, in other words find a way to be put right with God despite our sins.
The key question, though, is how are we justified? This was at the heart of the debate of the Reformation in the 16th Century. Martin Luther and the Protestants came to realise that the answer was simple, radical and joyful: We are justified by faith alone.