The Bible speaks of God overlooking centuries of ignorance of pagans who believed their idols were gods - Acts 17:30 - "but now he commands all people everywhere to repent." Then in 3:17 it speaks of the crucifixion of the Son of God and states to Jewish people:
"I know that you acted in ignorance, as did your leaders. But this is what God fulfilled what he had foretold through all the prophets, saying that his Christ would suffer. Repent, then, and turn to God, so that your sins may be wiped out."
God replaces ignorance with His truth, when He is bringing a person to salvation. He even uses the ignorance of some to fulfil His purposes. It's not so much a case of 'forgiving' ignorance, as providing the antidote to ignorance - the wisdom of Christ - and opening the eyes of those who He will save from their ignorance. It's a case of how God replaces darkness with light - the light of Christ, who is the Light of the World.
But it warns against wilful ignorance - knowing something (like the call to repentance) then disregarding it and carrying on as if what they knew was untrue, or not worth bothering about. (2 Peter 3:5)