Antecedent

antecedent: the word, phrase, or clause referred to by a pronoun. The AP exam occasionally asks for the antecedent of a given pronoun in a long complex sentence or in a group of sentences

"I look upon it as the climax of all misnomers..."
                                                  -Narrative of the Life of Frederick Douglass
                                                                    by Frederick Douglass

In this passage "it" refers to the "slave-holding religion of the land" being called Christianity. That is to say that the antecedent of "it" is the calling of the "slave-holding religion of the land" Christianity. Douglass uses the pronoun "it" for multiple reasons. By using "it" Douglass avoids tiresome repetition. If Douglass hadn't used the pronoun his book may have sounded like the story of a five year old (Daddy went to the store. Daddy took me to the store. Daddy bought me a toy at the store. Then Daddy drove me home). This reason is connected to other reasons. The use of "it" causes the prose to flow better. If Douglass had inserted "slave-holding religion of the land" every other sentence his prose would not have flowed, or been enjoyable to read.