Observation: As an example of mercy through Jesus, Paul uses his own example by giving Timothy a quick comment about his own life and ministry, information that Timothy very likely knows. He points out he received mercy and was deemed worthy of service once he had become faithful, even though previously he had been a terrible sinner. Then he says something that may be challenging for a couple reasons. Paul says he received this mercy explicitly because he "acted ignorantly in unbelief." This raises two issues that should not be considered lightly.
First, and perhaps very challenging to all, Paul is actively connecting the idea of "mercy" to "ignorance". This means mercy is available to those who sin when they are acting ignorantly in their unbelief. However, that could imply the inverse, that mercy is problematic for those who sin while living a life of belief, and thus with access to knowledge and wisdom. This is, of course, a major theological debate, at the core of faith vs. works, and already has terms, teachings, and two millennia of debate swirling around.
Second, and much more simplistic, this verse also shows that Paul is actively connecting the idea of "ignorance" and the idea of "belief". When it comes to life, and especially a life of belief in Yahweh as a Jew, Paul was absolutely never "ignorant". He was very likely one of the most well-educated of the Pharisees, the most educated class. He was the opposite of ignorant ... he was probably recognized as the gifted expert. However, Paul now ignores all his knowledge and uses only one measure to define wisdom versus ignorance, and that is belief in Jesus. When Paul was rejecting belief in Jesus, he was ignorant. There is no separation of these two conditions now in his mind.
Application: I am never going to know enough about the debate on faith, Christian behavior, and salvation to have any good opinion on this other than this very obvious statement: I believe in Jesus Christ as my Lord and savior, and every day I knowingly and sometimes willfully sin. For this, I pray for mercy and forgiveness, recognizing that - despite not being ignorant - I act ignorantly and do that which I hate. Therefore, I trust my faith is not conditional on my acts, else no one could be actually saved.
What I am more interested in, and able to wrap my head around, is the connection of ignorance and unbelief. I see wildly ignorant people all around me every day ... the ones who believe, promote, and champion such wild nonsense that reasonable discussions collapse before them because there is no logical foundation upon which they even stand. I see unbelievers around me every day ... the ones who either believe in strange ideas about how the universe operates, and/or actively reject any concept at all about a creator or even master architect that could be involved in life. And I know that, while there should be no real relationship between these two philosophical positions I just described, the Ven diagram between these two groups of people has a LOT of overlap.
Paul knew this. Paul knew that ignorance was unbelief (and vice versa). Belief doesn't just 'fix' ignorance, but unbelief is the garden where ignorance grows, and ignorance is the fertilizer that makes unbelief bloom.
Prayer: Lord, I am not ignorant. I know I behave ignorantly every day, and I do indeed pray for your love and forgiveness and mercy. Please forgive my sins, Lord. Also, please, let us find ways to introduce belief into more of this dark corner of the world, so many may be awakened from their ignorance. Amen.