It's usually not a positive or hopeful thing when you're told: "That will require a miracle" - but that's the sense of Jesus' words to Nicodemus.  Nicodemus wants to know how he can be assured a place in God's coming kingdom, he wants to check that's he's on the right track, doing all the right things under the Law.  However, Jesus floors him by saying: "Truly, truly I say to you, unless one is born again he cannot see the kingdom of God" (3:3).  Nicodemus is a good guy, but he's not good enough.  He needs a miracle - the change needed in His heart is so great it's akin to being born again!  Nicodemus is helpless and hopeless; just like the Israelites were in the wilderness being attacked by the snakes (3:13-15).  The snakes were God's judgement on their sin, and when bitten they would die.  They were helpless until Moses set up the bronze serpent on the pole, and in their helpless dying state they could only look to God's provision for rescue.  Jesus says that Moses and the lifting up of the serpent on the pole pointed to Him, who would be lifted up on a Cross of wood to provide rescue for helpless and hopeless people like Nicodemus.  We need a saviour and the wonderful news is that God is merciful towards sinners, as He is willing to save those who look to Him: "For God so loved the world that He gave us His son, so that whoever believes in Him should not perish but have everlasting life" (3:16).  We need a miracle and God is able to do the miracle in our hearts by His Spirit sent by Jesus to give us new hearts: "born of the water and the Spirit" (3:5) referring to "I will sprinkle clean water on you and you shall be clean...and I will give you a new heart and a new spirit I will put in you" (Ezekiel 36:25-26).