Further Elaboration Expected

 Something Significant

Jesus was, from everything I can tell, the first in all history to teach that each person had an unborn spirit within. This spirit had to be born, as it were like his body, from within. While the spirit of everyone in the flesh is unseen, the fruit of their spirit can be seen. This is how they are known. 

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While there are many spirits, clean and unclean, there is only one spirit known as the Holy Spirit, the Holy Spirit of God. 

No one taught this before Jesus' three year ministry and no one has made it a theme of their ministry since. Thus the phrase born again.