John 4:24

πνεῦμα ὁ θεός, καὶ τοὺς προσκυνοῦντας αὐτὸν ἐν πνεύματι καὶ ἀληθείᾳ δεῖ προσκυνεῖν.

God is spirit, and (for) those worshipping him, it is necessary to worship (him) in spirit and truth.


 * πνεῦμα ὁ θεός - the linking verb is omitted (a common feature). The subject is ὁ θεός as indicated by the article. πνεῦμα is a predicate nominative.
 * δεῖ - it is necessary. Typically takes a complementary infinitive, in this case προσκυνεῖν, to worship.
 * τοὺς προσκυνοῦντας - thus functions as a kind of "subject" of the verb προσκυνεῖν. These are the entities doing the worshipping. It is in the accusative, a feature known as "accusative of general reference". There is a similar English pattern, like I asked him to worship. The "him" is in the accusative and is the subject of the infinitive verb.