Douglass speaks about the ways in which slavery twists the ideas of what is decent and good. After his slave mistress, Mrs. Auld, teaches him the rudiments of reading, her husband, who has far more experience as a slave master than she does, roundly criticizes her. He says that reading will forever make a slave unfit for slavery.

Irony involves the use of something that is contrary to what one expects. Reading is a positive good, as is education, but in the slave-holding South, teaching someone to read becomes evil and even an offense, as it makes slaves begin to question their inhumane conditions and perhaps to want to escape. Douglass also uses the word "Christian" to show the irony of the twisted and inhumane way slaves are treated. A society in which teaching someone to read is an unjust society.

Irony is used here because really, teaching someone to read would be very consistent with Christianity.  So when Douglass says that it is unpardonable, that's very much against what would be true in a really Christian country.

The ability to read is pretty basic to Christianity because it allows a person to read the word of God.  Since all people are equal before God, and God wants them to come to Him, it is important to teach them to read so they can understand His word for themselves.

Therefore, in some ways, preventing people from learning to read prevents them from fully understanding God's word.  This goes against true Christian values.