Here is another excerpt from the classic book about what all Christians believe, Mere Christianity, by C. S. Lewis.
The first thing to get clear about Christian morality between man and man is that in this department Christ did not come to preach any brand new morality. The Golden Rule of the New Testament (Do as you would be done by) is a summing up of what every one, at bottom, had always known to be right. Really great moral teachers never do introduce new moralities: it is quacks and cranks who do that. As Dr. Johnson said, 'People need to be reminded more often than they need to be instructed.' The real job of every moral teacher is to keep on bringing us back, time after time, to the old simple principles which we are all so anxious not to see; like bringing a horse back and back to the fence it has refused to jump or bringing a child back and back to the bit in its lesson that it wants to shirk.