Jesus to Nicodemus or the writer John to the readers.

Compare with other instances of this by John:

30 Many indeed, therefore, other signs also did Jesus before his disciples, that are not written in this book;
31 and these have been written that ye may believe that Jesus is the Christ, the Son of God, and that believing ye may have life in his name.` Jn 20
1 After these things did Jesus manifest himself again to the disciples on the sea of Tiberias, and he did manifest himself thus: Jn 21

36 For these things came to pass, that the Writing may be fulfilled, `A bone of him shall not be broken;`
37 and again another Writing saith, `They shall look to him whom they did pierce.`
38 And after these things did Joseph of Arimathea -- being a disciple of Jesus, but concealed, through the fear of the Jews -- ask of Pilate, that he may take away the body of Jesus, and Pilate gave leave; he came, therefore, and took away the body of Jesus, Jn 19

70 Jesus answered them, `Did not I choose you -- the twelve? and of you -- one is a devil.
71 And he spake of Judas, Simon`s son, Iscariot, for he was about to deliver him up, being one of the twelve. Jn 6
1 And Jesus was walking after these things in Galilee, for he did not wish to walk in Judea, because the Jews were seeking to kill him, Jn 7

46 for if ye were believing Moses, ye would have been believing me, for he wrote concerning me;
47 but if his writings ye believe not, how shall ye believe my sayings?` Jn 5
1 After these things Jesus went away beyond the sea of Galilee (of Tiberias), Jn 6

54 this again a second sign did Jesus, having come out of Judea to Galilee. Jn 4
1 After these things there was a feast of the Jews, and Jesus went up to Jerusalem, Jn 5

 

They're definitely the words of Christ as the conversation isn't abruptly ended in this passage. I've never though of it any other way, nor have I been taught that is anything other than the word of Christ.

 

That's a difficult passage to know where to put the quotation marks, and also if the target language has honorifics. Where do you apply the honorifics, and where is the passage narration? Very difficult to tell.

 

The honorific treatment of the words of Jesus would then stop at v. 21. So, the main problem is not the honorifics per se, but where to start and stop honorifics.

Just looked at our John 3, and v. 21 is actually where we stopped the quote of Jesus.

 

Whether they be words of Christ or John's narrative, both are equally true, right?

Ditto.

 

One difficulty is that Jesus frequently talked about himself in the third person. Another is that the Jesus's speech in the Gospel of John appears in the same style as narrator's. Read John 1:1–18 and then read John 3:16–21. It might as well be the same person talking.

Interestingly, the narrative transition marker "after this/after these things" (μετα τουτο/ταυτα), which another poster already mentioned, usually comes after a comment by the narrator (8x; cf. 2:12; 5:1, 14; 7:1; 11:7; 19:28, 38; 21:1). The exception is 6:1, since 11:11 and 13:6 are different linguistic uses of the expression. I don't think this frequency is enough to assert that the comments before the expression in 3:22 are necessarily by the narrator. However, it is interesting how Nicodemus trickles off and then simply disappears from the dialogue in John 3. Enough interpreters have wondered if the narrator did the same with Jesus.

As for translations, Weymouth, NIV (2011), and the NET Bible end Jesus's words after 3:15, and Today's English Version ends them after 3:13. Wherever one lands on the issue, nothing is really affected unless, as the old liberals, one applies more significance to the ipsissima verba of Jesus than to the words of the narrators.