When Christ referred to the 'beginning', He was asking his questioners (and us), to go beyond the boundary between the state of original innocence and sinfulness.
Symbolically this boundary can be linked to the Tree of the Knowledge of Good and Evil. It delimits two diametrically opposed states. These states have a specific dimension in man: in his inner self, in his knowledge, conscience, choice and decision. And all of this is in relation to God the Creator who is also God of the covenant(s). The tree ties the two states together, but also, with Christ instructing us to *before* the 'incident of the tree', He is highlighting the connection between the two states. The continuity of mankind.
Being sinful is a part of mankind. We cannot be rid of it. We have inherited a sinful nature - there is something within us that is inclined to sin. It's easier than being good. We're tarnished, but we are still, at our core, good - or there would be nothing to redeem. Mankind, represented by Adam and Eve, broke the first covenant, but then, they also entered into the second. They participated both in our downfall, and the seeds of our redemption.
'In the same way, therefore, historical man - both Christ's questioner at that time, of whom Matthew 19 speaks, and modern man - participates in this perspective. He participates not only in the history of human sinfulness, as a hereditary and at the same time personal and unique subject of this history; he also participates in the history of salvation, here, too, as its subject and co-creator. Therefore, he is not only closed, because of his sinfulness, with regard to original innocence, but is at the same time open to the mystery of redemption, which was accomplished in Christ and through Christ.'