The agent of an infinitive (when it is expressed) will be in the accusative case (GGBB p. 192).
Καὶ καθὼς Μωϋσῆς ὕψωσεν τὸν ὄφιν ἐν τῇ ἐρήμῳ οὕτως ὑψωθῆναι δεῖ τὸν υἱὸν τοῦ ἀνθρώπου, (John 3:14)
In this verse, υἱὸν is the agent of the infinitive ὑψωθῆναι.
