Theory:

     The moon was above, peaceful and bright. There were few people passing. He could hear birds high above him. And the music that came from the room held Soapy there, for he had known it well long ago. In those days, his life contained such things as mothers and flowers and high hopes and friends and clean thoughts and clean clothes.

     There was a sudden and wonderful change in his soul. He saw with sick fear how he had fallen. He saw his worthless days, his wrong desires, his dead hopes, the lost power of his mind.

     And also, in a moment, his heart answered this change in his soul. He would fight to change his life. He would pull himself up, out of the mud. He would make a man of himself again.
Explanation:
 
As it was night time, Soapy could see the moon above the sky. The moon seemed peaceful and bright. Few people were passing here and there near the country churchyard. Soapy could hear the chirping sounds of the birds sitting on the edges of the roof. The music made him to stop there and he had known it well long ago.
 
moon-2913221_1920.jpg
A peaceful moon
 
The music was nothing but "Sabbath anthem".* Anthem is a religious song sung by a choir with organ music. The anthem was being played by an organist inside the living room and Soapy heard that sweet music coming through the window. The anthem made Soapy to think of his past. During his childhood, his life contained many good things like mothers, flowers, high hopes, friends, clean clothes and even clean thoughts.
 
In our story "The Cop and the Anthem", the term "anthem" plays an important role because it gave a big transformation in Soapy's spirit. The Anthem had made a sudden and unexpected change in his spirit. He recalled the vast transformation that he had in his life. Soapy had gone through a lot of meaningless days and had a lot of bad wishes, like getting into the prison by committing a sin. His dead hopes and lost power of mind had made him to fall into the pit.
 
While thinking about his bad transformations, his heart had answered him a way to change his life. It instructed him to fight to change his life. Here fight doesn't mean violence, it speaks about the hard work. If a person works hard he would come up in his life. Soapy would pull himself out from the mud. Here, mud implies the wrong desires, wrongs hopes and wrong intentions he had in his life. Soapy would come out from his worst part of life and would start a new life. Because of the music, he decided that he wanted to be a good person in his life.
 
Meanings of the difficult words:
 
S.No
Words 
Meanings 
1
Worthless Having no real value or use
2
DesireA strong feeling of wanting to have something or wishing for something to happen
3
MudSoft, sticky matter resulting from the mixing of earth and water
4
HopeA feeling of expectation and desire for a particular thing to happen
Reference:
National Council of Educational Research and Training (2007). An Alien Hand. The Cop and the Anthem- O. Henry(20-29). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.