This is an extremely dangerous error. Sin, or a "sinful nature," never made anyone "fully human". It's basically the lie the serpent told Eve in the garden -- until you sin, you aren't really human.
It's rare for me to find an actual interesting point on this "Religion Forum". This, however ...
Very interesting, indeed.
Jesus DID NOT have a "sinful nature". He can't have, He's GOD. He's also truly Man ... more fully Man than any of us can hope to be. Furthermore, when Adam and Eve were created, they did not have a "sinful nature". Quite the contrary; when God created them, He found them to be "very good".
Thus, Sin is not part of our Human Nature. It was not present at the beginning, and not present in the Man Jesus Christ. Rather, sin is a defect in our Human Nature. It is not something God inflicted on us. It is the result of Adam and Eve (and the rest of us) breaking what God gave us.