Just about the most recorded song ever. Yes for most listeners, sadly, it's a moving tune with fairy-tale type words. As one of the YouTube listeners wrote "I may not be Christian, but this song just makes me smile and cry at the same time."
But in reality, its the Gospel message written by John Newton, the worst of sinners, who saw the light and went on to spread God's word by his preaching and plethora of hymns.