Nicodemus could hardly believe such a thing. All his life he had been taught that good works were the key to heaven - that if a person observed all the laws and requirements of God, he would have entrance into heaven. But Jesus was talking about things of the spirit. The problem was that Nicodemus, like all human beings, was of the flesh. He was from the earth.
Flesh is mortal.
It is subject to death, decay, and corruption. Many people try everything imaginable to ignore or forget this, but the fact always remains that we dwell in a mortal body which will soon return to dust. The Answer Our old state, our old nature, and even all our good deeds are insufficient. God is totally and absolutely holy and righteous. He is perfect.