This question is based on four texts dealing with this matter.
John 3:3 where Jesus said to Nicodemus, "Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God."
John 6:40 where Jesus promised that "every one which seeth the Son, and believeth on him, may have everlasting life, and I will raise him up at the last day."
John 20:29 where Jesus said to Thomas, "Thomas, because thou hast seen me, thou hast believed; blessed are they that have not seen, and yet have believed."
1 Peter 1:8, speaking about believing Jesus, "Whom having not seen, ye love; in whom though now ye see him not, yet believing, ye rejoice with joy unspeakable and full of glory."
All from the A.V. and bold emphases mine.
What kind of 'seeing' is this? It's clearly not seeing material things (like a literal kingdom on earth, or a person visible to our eyes.) Though people at first did literally see Jesus and believed in him, he spoke of those not seeing him, yet being begotten again' with a heavenly inheritance (1 Peter 1:3-4).
Only those who believe what Jesus said about being 'born again' as an adult (as he said to Nicodemus) are being asked, which effectively rules out many Christians who believe babies or little children are 'born again' when baptized as such. In other words, answers are not sought from those who equate membership in a religious organization with being born again and/or seeing the kingdom of God.
So, what kind of 'seeing' is this, and how is 'seeing Jesus' key to this?