{Moses lifted up the serpent} (M(9375)s(8873) hups(9373)en ton ophin).
Reference to  Nu 21:7ff.  where Moses set the brazen serpent upon
the standard that those who believed might look and live. Jesus
draws a vivid parallel between the act of Moses and the Cross on
which he himself (the Son of man) "must" (dei, one of the
heavenly things) "be lifted up" (hups(9374)h(886e)ai, first aorist
passive infinitive of hupso(935c), a word not used about the brazen
serpent). In John hupso(935c) always refers to the Cross ( 8:28  12:32,34 ), though to the Ascension in Acts ( Ac 2:33  5:31 ).
Jesus is complimenting the standing and intelligence of Nicodemus
as "the teacher of Israel" by telling him this great truth and
fact that lies at the basis of the work of the kingdom of God
(the atoning death of Christ on the Cross).