The central Christian belief is that Christ's death has somehow put us right with God and given us a fresh start. Theories as to how it did this are another matter. A good many different theories have been held as to how it works; what all Christians are agreed on is that it does work.
Everyone of us wakes up in the morning, goes to the bathroom, looks in the mirror and asks: "Who am I? Who am I today? Do I feel good enough? Do I feel big enough? Do I feel sexy enough?" Some days, the answer is 'yes' but sometimes it's not.