Jesus indeed told the Pharisee, Nicodemus, in John 3:3, "Most assuredly, I say to you, unless one is born again, he cannot see the kingdom of God."
Some, perhaps many, modern professing American Christians and others around the world, say this. Indeed, perhaps many were born again at one time or believe they were!
This post really gets to the heart of the "once-saved, always saved" doctrine held by many Christians.
In order to maintain consistency, many Christians say that a "fallen away' Christian was never saved at all!
One of the key passages that seems to support this view, is found in Matthew 7:21-23.
This passage indicates that demonstrations of power and confessions of Jesus' name do not guarantee that a person knows God or ever knew God, therefore not being truly "born again."
In Matthew 7:21, Jesus said, "Not everyone who says to me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven." Jesus, amazingly details those who have cast out demons in His name, prophesied in His name and done many wonders, again in His name! However, Jesus replies, "I never knew you; depart from Me' you who practice lawlessness." It seems that the Lord God demands that professions of faith or proven in word, deed and heart condition.
This passage almost causes one to be sympathetic to those being denied eternal life and told, "...depart from Me..." by Jesus. It is almost like these people are blindsided and claim that the Lord is being grossly unfair, like He is playing a "head game" with such people. Much like how we treated pledges in my college fraternity! We made it so there was no way the plebes could win in their Hell Week and you get the same kind of hopeless disgust and despair from these people!
But the analogy is not accurate! The folks in Matthew 7:21-23 were liars and frauds, ultimately they were more about personal gain and getting the world's approval that truly doing God's will. They were about outward expressions of piety (ala the scribes and Pharisees in Matthew 23:5-7 who "...love the best places at feasts...) and not about a transformed, obedient heart that marks Jesus' true disciples (John 10:14, I Corinthians 8:3, Ephesians 3:17 and Revelation 3:17-13). I Corinthians 8:3 notes, "But if anyone loves God, this one is known by Him."
Personally, this passage amazed me and I thought, who are such people? After closer study (knowableword.com has an excellent set of examples in its post, "Context Matters: I Never Knew You; Depart From Me" from April 26, 2019 by Peter Krol), the Bible provides many examples!
Perhaps the best is Jesus' own disciple, Judas Iscariot! Judas was authorized, along with the other eleven disciples by Jesus "to cast out demons", among other good works in Mark 3:14-19. Judas exposed himself ultimately in being more interested in worldly gain than following Jesus with his betrayal. Judas greeted Jesus with, "Greetings, Rabbi." Ultimately, Judas was a "bad egg," a phony, a thief, a Devil!
We also find other examples in the Bible, like:
*Balaam, the Old Testament prophet, who initially blessed Israel in Numbers 23-24 but ended poorly in Numbers 31:8 and 16.
*King Saul, ultimately rejected by God, for disobedience, was refreshed when David played the harp in I Samuel 16:23.
*Demas, one of Paul's assistants, left Paul, "...having love this present world..." in II Timothy 4:10.
*those in Isaiah 29:13 who honored God with their lips but not their lips.
*....and the foolish virgins in Matthew 25:11, who were not prepared for the coming of the groomsmen!