So, working on my sermon for Trinity Sunday and having a look at John 3:1-17, with special attention to verses 16 and 17, I noticed in the Greek some interesting parallelism, which I will try to capture in English (adjusted to reflect the underlying Greek so as to highlight preciseness of parallels).
We start first with a set of parallels which includes v. 15.
1. Parallel concerning the ones believing in Christ.
Verse 15 is: "in order that everyone believing in him might have life eternal."
Verse 16 includes: "in order that everyone believing in him might not perish but might have life eternal."
2. Parallel concerning God's action on or to the Son:
Verse 16: God ... the son the only gave
Verse 17: Not for sent God the son
In each case the world (see below) is in view as the reason why God "gives" the only Son and why God "sends" the son.
We deduce that in both verses John is reporting something similar: for the sake of the world God involves the Son (via theologically weighted words, "gave" and "sent") in the salvation of the world.
The theological weight on "gave" includes Abraham being willing to give his only son as a sacrifice (Genesis 22), Mark's report of Jesus talking about giving his life as a ransom (Mark 10:45), and Paul's reference to God not withholding "his own Son, but gave him up for all of us" (Romans 8:32). And for "sent" we can read the whole of John's Gospel as a story of God's agency: Jesus sent to the world, Jesus coming into the world in order to reveal, to bring abundant life via signs and speeches, and, ultimately, to save the world through taking away the sins of the world.
The next observation is less about a parallel and more about
3. development of God's attitude and action in respect of the world:
Verse 16: ... for loved God the world
Verse 17: not for sent God the son into the world in order that might be judged the world but in order that might be saved the world through him.
Note there is a parallel between God's actions: loved ... the world, sent ... into the world. God's love acts, and God's acting is loving.
(We might additionally note that in verse 18 the theme of "judge(d)" is developed in relation to "believe" and also "only son":
Verse 18: The one believing in him is not judged but the one not believing has been already judged, because he has not believed in the name of the only son of God.)
As John reports Jesus' (Aramaic) discourse in Greek, John works his literary magic to emphasise and develop themes such as believing, world, and judging, in relation to God's love for the world - a love which both "gave" and "sent" the son.
John uses parallelism, repetition (of individual words, of phrases, of themes expressed previously) to underscore the importance of God's initiative out of love for the world and the appropriate response of believing in the son.
John the theologian is not only transmitting the teaching of Jesus, he is transforming it when translating it in order to convey the key messages he sees at the heart of the gospel, a "seeing" which he credits to the Spirit of Truth working in the disciples of Jesus after the exaltation of Jesus in order to develop the fullness of the meaning of Jesus' teaching (so John 14-16).