Before John wrote, Jesus explained it, as in Mark 10:15, Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.
Nicodemus' question still applies, . . . How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?
It is the same answer.
1 John 5:1, Whosoever believeth that Jesus is the Christ is born of God.
The Lord explains how men can be born from above/born of the Spirit/born again, and this in response to Nicodemus' question:
John 3:9
9 Nicodemus answered and said unto him, How can these things be?
John 3:14-15
14 And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up:
15 That whosoever believeth in him should not perish, but have eternal life.
The Son of Man's lifting up, that is, His death in our stead—is the means of Regeneration.
The fact that God sent His Son that men might have everlasting life points out that prior to the Son of God being sent no man had everlasting life. Men were justified, but not regenerated. Regeneration requires the eternal indwelling of God, and this did not begin until Christ returned to Heaven:
John 14
16 And I will pray the Father, and he shall give you another Comforter,
that he may abide with you for ever;
17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but
ye know him; for he dwelleth with you, and shall be in you.
This makes it clear that the disciples were not yet eternally indwelt. They knew (of) the Spirit, for He dwelt with them (and they rejected Him more often than not, Acts 7:51), but "in that day," He would be in them.
Christ's death was necessary for the redemption of our sins, and His resurrection necessary to provide for Eternal Life through God's eternal indwelling.
John 16:7-8
King James Version
7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.
8 And when he is come, he will reprove the world of sin, and of righteousness, and of judgment:
The eternal indwelling began when the Christ returned to Heaven and the Comforter, the Spirit of Truth was sent on the Day of Pentecost. Another indication of the disciples unregenerate state is seen here:
Acts 1:4-5
King James Version
4 And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me.
5 For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.
While some view the Baptism with the Holy Ghost as a "second blessing," we can dismiss that as a valid position simply by acknowledging that God, many times, empowered Old Testament Saints. What He did not do is indwell them on an eternal basis, because their sins had not yet been redeemed by the death of Christ in the stead of the sinner. We see the disciples go from being a committee in a locked room tossing dice to make decisions to Gospel evangelists when they are born again through the coming of the Comforter.
This does not mean the Old Testament Saints were not "saved," it simply means they were saved according to the ministry of God in those Eras. Just as we are saved yet not glorified, even so they were saved but not yet regenerate.
One last passage that shows Eternal Redemption was accomplished by Christ and that the sins of the Old Testament Saints were redeemed by His death:
Hebrews 9:12-15
King James Version
12 Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.
13 For if the blood of bulls and of goats, and the ashes of an heifer sprinkling the unclean, sanctifieth to the purifying of the flesh:
14 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
15 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
Going back to John 3, Nicodemus asks "How can these things be?" The answer should be simple for all of us on this side of the revelation of the Mystery of the Gospel of Christ: Only by the Sacrifice of Christ can we be reconciled to God and brought into an eternal union through the process of receiving His life when we are immersed (baptized) into Him.
God bless.