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.