This gets driven home not only in the necessity of starting afresh after Confession but also with the transmission of basic truths about Jesus. The blood, sweat, prayers and study of so many before us are accounted little; the very nature of religious education is to relearn what previous generations knew. History shows the need to reinvent the wheel never diminishes. No wonder in 2 Peter the sacred author writes, "I will always remind you of these things, even though you already know them and are established in the truth you have.". There's no resting on the laurels of truth given the assaults of the evil one and human lack of faith.
It can seem discouraging that whole continents (like Europe) will need to metanoia and start over - but much, much less so when one realizes it happens on a micro scale in us and that's the way it's always been. Fortunately God is patient, and if he can put up with our constant need to re-learn and turn around than we can't be impatient with the same. I admire the cheerfulness of St. Ignatius of Loyola whose feast day was not long ago. From a meditation on his life:
"Soon after their foundation the Jesuits began to meet the challenge of the Reformation: a tough task, given the debilitated state into which the Church had fallen, but one which, as Ignatius said, had to be undertaken 'without hard words or contempt for people’s errors'."