Anamnesis is a philososopical term used by Plato. It is the idea that humans possess knowledge from past incarnations and that learning consists of discoving this innate knowledge. In the Meno, Socrates, by asking a series of questions, demonstrates that virtue is knowledge.
I would say the idea here, setting aside reincarnation, is that a person has an innate knowledge of virtue. This is also known as the Platonic Form. A disturbance of or in this source of knowledge is then a psychological problem. In modern terminology, it would be something like sociopathy where a sociopath does not know right from wrong. It is difficult to see how civilization and religion could even have developed if humans did not have this innate knowledge of right and wrong. In that way, conscience would be a factor before even the earliest beginnings of human history. Perhaps this is also what Ratzinger meant by anamnesis.
It is very difficult to see, for example, how the ancient Greeks or the chosen people of the Law, centuries prior to Christianity and Church teaching, did not know right from wrong. Really, the notion is absurd. It is not as though what always was, is now and ever shall be did not exist in the pre-Christian world. What then is the pre-Christian world? Since Christ is God, He who always was, is now and ever shall be, could there have been such a thing?These are all human concepts or ideas and the product of learning, and maybe this is not so easy to follow at first glance. The earth, after all, is God’s Creation.
‘Certain’ knowledge in English means fixed or settled, where there is no doubt. It is not the same thing, however, as objective knowledge. It is from the interior, or the soul. What I see as important, however, is to beware of righteous judgments proclaiming with supposed certitude to know the absolute truth for you or me.