John 3:11-12, 11 Most assuredly, I say to you, We speak what We know and testify what We have seen, … 12 If I have told you earthly things and you do not believe, how will you believe if I tell you heavenly things?
Nicodemus a teacher of the law was in the natural, the spiritual things made no sense to him, he could not discern them. If he had trouble understanding the earthy, how was he going to get the matters of heaven unless it was revelead to him. Jesus asked the disciples of what men thought of who He was, Peter as Jesus said confessed Him by revelation.
Understanding is revelation God gives. Luk 24:45, “Then he opened their minds so they could understand the Scriptures” (NIV). If it’s understanding we need, then the question is not “WHAT” brings us this understanding but “WHO”, 1Joh 5:20 “And we know that the Son of God has come and has given us understanding, that we may known Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life”. He who has the Son by revelation has eternal life.