AUL taught that all humanity is trapped under a thick blanket of spiritual darkness. Truly, it was our own foolish choice to enter this darkness (think Adam and the forbidden fruit), yet the entrapment is still tragic. Paul was particularly saddened by the position of ignorance in which his own Hebrew people stood. God had given them so very much. Yet in ignorance, the Jews were very zealously attempting to establish their own spiritual resume.
There is another category of ignorance which could be dubbed "impotent ignorance" (vs. 14 & 18). There are two reasons for this kind of spiritual ignorance. On the one hand, it is a result of a lack of spiritual instructors (or instructions). On the other hand, on some level God has revealed Himself to everyone. Still, due to an inability to do anything about it, the ignorance of this group continues. They might know enough to know that there is more to be known, but they don't know how to go about discovering it.
The third and final category of ignorance dealt with here is this so-called "innocent" ignorance. I do not mean to use this word "innocent" in any sort of an absolute moral sense. The gentiles are lumped together in a category that we might call the irrelevant. We were on the outside looking in. More accurately, we were on the outside and were not even aware that there was anything to investigate. So, perhaps we should say uninvolved. We were bystanders in a program involving God and His chosen people. We were ignorant of His plan, ignorant of His mercy, and ignorant about our own condition (Acts 17:30 & Romans 10:20).