"That which is born of the flesh is flesh".
That's right and Jesus was born of flesh by being made of a woman. And the apostle John says that anyone who would deny that Jesus had come in the flesh is a deceiver and an antichrist.
The NIV translates the word 'sarx', which means 'flesh', as sinful nature. The Greek says 'flesh of sin'.
The RCC piles up one false doctrine on another. That's how it goes when you begin with lies they pile up one on the other. After the RCC decided Jesus was not of the same flesh as his brethren they invented a doctrine they call "the immaculate conception" whereby it make Jesus' mother to be of a different flesh as the rest of mankind so that she could give birth to the same type of creature that she became.
Now the RCC, with it's anticrist doctrine, has deceived the vast majority of "Christianity" and created billions of antichrist.