It works like this.
Christ is truly God and truly man. This we know from Nicaea. What this means is that he is both - at the same time.
For him to be the Son of Man, and truly man, he must be born - of a woman. What this also means is that he would be stained with original sin.
There’s no way around this, unless you state that he wasn’t born of a Virgin, but rather born ‘in’ a Virgin.
The implications of in vs of are enormous. For one, it denies the incarnation. Christ wasn’t truly man, because he did not inherit a human nature. He was God - putting on a human form. It fabricates the death on the cross.
So, if he’s born of a Virgin, and gets his human nature from Mary, why didn’t Christ have original sin? Why is he both linked to Adam (as in the genealogy in Luke), yet is entirely free from sin?
Well, that means that he must have been born of a Virgin who was, herself, sinless. But we know that no one, but God can be sinless.
It all links together. Mary’s immaculate conception, her sinlessness, her assumption (because those who are sinless cannot die), etc. and it all comes about because Christ is free, not just from his own sin, but also from original sin AND has a fully human nature, just like us. He is more man than we, who have fallen since the fall.
But then, how did Mary not be stained by sin? I thought that Bible says that sin is passed by the seed of man? Mary’s dad would have had to have been sinless for Mary to be sinless. However since Christ was born of a virgin by the hand of the holy spirit, then Christ, not being of the seed of man would have been sinless in the flesh.
That is why the Virgin birth was so important.