This is part of what has been long revealed. The doctrines that Christ is here presenting are divided into two parts.
What precisely did Christ refer to in this verse – ‘If I have told you earthly things’? He means, if I have told you of things that take place on earth and you think this is too astonishing to be true – a new birth by God. How can you believe if I tell you the heavenly side? If we do not have faith to believe the works of God on earth, you certainly do not have faith to believe the mysteries of heaven, for faith is the only handle we have on things that are hidden from us and must be received by divine testimony. Let the unconverted be careful not to overestimate their capacity to grasp the deep mysteries of God. Christ deals perfectly with Nicodemus in his ignorance of his own state before God.