“In very truth I tell you, we speak what we know, and bear witness of what we have seen, and you do not receive our testimony. If I have told you earthly things and you do not believe, how will you believe if I tell you heavenly things?”
‘We' is Jesus and His disciples. ‘You' (plural) are Nicodemus and his co-religionists. ‘We (Jesus and His disciples) speak of what we know, and testify to what we have seen'. Already such things have happened in the ministry of Jesus that they should have convinced the world. Many lives have been transformed, many men have become more dedicated to God. Wonderful things are happening on earth. And they should have observed them.
And then he adds, ‘but you (plural) do not receive our testimony'. Here Jesus links Nicodemus with his co-leaders. The authorities had come to observe and to criticise, but they were not spiritually perceptive enough to recognise what was happening, that the promises of the prophets about the Spirit being poured down were being fulfilled. (While the greater inundation is in the future, the Spirit's present work, constantly referred to in John's Gospel, should have been sufficient to convince even the most incredulous).
This being so, even if He tells them even more wonderful things, He doubts if they will accept them, for they are deliberately closing their eyes. ‘If I have told you earthly things, and you believe not, how can you believe if I tell you heavenly things?' Those who will not accept the evidence of God's activity before their eyes on earth, cannot hope to appreciate the deeper facts which His coming has brought into play. The things that are happening have far deeper roots than what is obvious on the surface, but if they are to be appreciated they require a total rethink.