It seems that what caused Nicodemus to stumble in our gospel was not so much that he did not understand what Jesus was saying, but the impracticality of His thought. To be born again may sound good, but the practical implications of such a thought are far reaching.
Nicodemus was old; he had lost heart; his fight seemed over; for him things had become ruthlessly and inexorably fixed. There was no turning back to live things over again, and this time better. What is once done cannot be undone and Nicodemus was painfully aware of that.
Slowly, unconsciously, inevitably, act by act, thought by thought we, like Nicodemus, have built up a personality. And now we are closely immured in it. There is no escape. We are left to muse with Nicodemus, "How can a man be born when he is old? - if not old in years, then old in sin?" This is what I have made of life; and it cannot be changed now.
But the whole point of the gospel is that God can do just that for anyone; for you and me. The gift of the Spirit shakes a person's life to its roots. It overcomes all opposition; no matter how great a fortress we have built around ourselves; no matter how ingrained our habits and patterns of behavior may be.
We don't re-enter our mothers' wombs, but come forth from the womb of mother Church: born to new life through baptism and nursed by her - feeding upon the milk of the body and blood of our Lord. We need not even wonder or be anxious about how this happens, Jesus tells us; no more than an infant wonders at the care his mother provides. Ours is but to trust in the One who loves us and has given us new life.