Jesus was telling Nicodemus about a SPIRITUAL birth, He couldn't have been clearer:
Jesus answered, “Very truly I tell you, no one can enter the kingdom of God unless they are born of water and the Spirit. Flesh gives birth to flesh, but the Spirit gives birth to spirit. You should not be surprised at my saying, ‘You must be born again.’ The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit.” (John 3:5-8)
The Lord God spoke of it to the Jews as in Ezekiel 36:26,
I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh.
And as Paul said in Titus 3:5, he saved us, not because of righteous things we had done, but because of his mercy. He saved us through the washing of rebirth and renewal by the Holy Spirit.
Add to these verses and others such as Ephesians 1:13, Ephesians 4:20 and 2 Corinthians 1:22 that tell us we are sealed with the Holy Spirit right here and now through faith, and you can understand that we certainly ARE given assurance of our salvation and the indwelling Holy Spirit IS our guarantee.
The birth into the incorruptible body is the spiritual birth.
It has not yet happened to anyone but Yeshua.
Your lack of understanding on this in no way surprises me.
WE are not presently equipped for existence in the Kingdom; corruptible flesh cannot enter therein; that is what Yeshua was discussing with Nicodemus, who was already well educated in all things spiritual, having been educated in the gospel as taught by Moses from birth. Paul also explained this (1Cor 15:51-54)
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