That's funny, Jesus told Nicodemus he must be born again before He went to the cross.
One Lord, one faith, one baptism. The power of the Holy Spirit is all that came and left in the OT.
IMO, people were born again in the OT, just like the new. Few were saved before the Spirit was poured out at Pentecost. If this is not true, how did they worship God in the OT. How did they understand Scripture? How did they love God and their neighbor? A lot of questions arise when we do not see God working the same way in the OT. And we have Peter that tells us Christ was indwelling the OT Prophets. Also Paul says people were born again in the OT. Most people don't understand this because that is what we are taught.
10As to this salvation, the prophets who prophesied of the grace that
would come to you made careful searches and inquiries,
11seeking to know what person or time the Spirit of Christ within them was indicating as He predicted the sufferings of Christ and the glories to follow.
12It was revealed to them that they were not serving themselves, but you, in these things which now have been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven—things into which angels long to look.
Gal 4,
28And you brethren, like Isaac, are children of promise.
29But as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so it is now also.
When Jesus was walking around on the earth in the flesh, He could not come and dwell in a believer by the Holy Spirit. But afterward He could, this was a unique time frame. IMO, that answers John 14. Jesus was abiding with the disciples, He will then come back through the Holy Spirit and indwells them., never leaving them.
16“I will ask the Father, and He will give you another Helper, that He may be with you forever;
17that is the Spirit of truth, whom the world cannot receive, because it does not see Him or know Him,
but you know Him because He abides with you and will be in you.
18“I will not leave you as orphans;
I will come to you. 19“After a little while the world will no longer see Me, but you
will see Me; because I live, you will live also.
20“In that day you will know that I am in My Father, and you in Me, and I in you.
21“He who has My commandments and keeps them is the one who loves Me; and he who loves Me will be loved by My Father, and I will love him and will disclose Myself to him.”
22Judas (not Iscariot) said to Him, “Lord, what then has happened that You are going to disclose Yourself to us and not to the world?”
23Jesus answered and said to him, “If anyone loves Me, he will keep My word; and My Father will love him, and We will come to him and make Our abode with him.
24“He who does not love Me does not keep My words; and the word which you hear is not Mine, but the Father’s who sent Me.
25“These things I have spoken to you
while abiding with you. 26“But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you