From Parochial and Plain Sermons
by Venerable John Henry Newman:

When Nicodemus doubted about our Lord's declaration, that a birth through the Spirit was the entrance into His kingdom, He said, "If I have told you earthly things, and human nature." Thus the greater Mystery of the Incarnation is made to envelope and pledge to us the mystery of the new birth. As He was in Heaven in an ineffable sense, even "in the days of His flesh," so are we, in our degree; according to the words of His prayer, that His disciples might "all be one; as ThouWhen Nicodemus doubted about our Lord's declaration, that a birth through the Spirit was the entrance into His kingdom, He said, "If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things? And no man hath ascended up to Heaven, but He that came down from Heaven, even the Son of man which is in Heaven." In these words our Lord plainly discloses that in some mysterious way He, the Son of man, was really in Heaven, even while, by human eyes, He was seen to be on earth. His discourse seems to run thus:—"Are you offended at the doctrine of the new birth of the soul into the kingdom of God? High as it is, it is but an earthly truth compared with others I, as coming from Heaven, could disclose. It is mysterious how regenerate man should be a citizen of a heavenly kingdom, but I Myself, who speak, am at this moment in Heaven too, even in this My human nature." Thus the greater Mystery of the Incarnation is made to envelope and pledge to us the mystery of the new birth. As He was in Heaven in an ineffable sense, even "in the days of His flesh," so are we, in our degree; according to the words of His prayer, that His disciples might "all be one; as Thou, Father, art in Me, and I in Thee, that they also may be one in Us." [John xvii. 21.]