All ancient bible teaching Cofty; although not necessary. Just reading the bible and believing it leads to the same conclusion.
This verse is connected to what Jesus had been telling Nicodemus, although the connection is not immediately obvious. The verse is cited in the Ante-Nicene commentaries of Tertullian, Hippolytus, Novatian, Archelaus, and is also included in the ancient Syriac Diatessaron of Tatian.
Nicodemus understood Christ to be a great teacher and perhaps a prophet because of the miracles he had performed (John 3:2). In verse 13, Jesus is extending his dialog with Nicodemus to reveal that He is much more than a prophet. No mere man, he says, hath ever ascended up to heaven as he will.
Theophylact (11th century Byzantine commentator) summarizes the Patristic interpretation of this verse as follows:
Because Nicodemus thought Jesus was a teacher and prophet, the Lord now says to him, "Do not imagine that I am an earthly prophet sent by God. I came down from heaven as Son of God, and I am not from the earth. No prophet hath ascended up to heaven.* I alone shall ascent, as I descended."
When you hear that the Son of man came down from heaven, do not imagine that flesh came down from heaven. (Apollinarius taught this heresy: Christ came down from heaven in the body and entered the world through the Virgin as through a channel.) Since Christ has two natures united in one hypostasis, or person, the names that refer to His human nature may also be addressed to God the Word; conversely, the names that refer to the divine Word may be addressed to Christ as man. Thus, in this verse, Christ calls Himself the Son of man, Who came down from heaven.
The Lord adds the words Who is in heaven for a specific reason: "When you hear Me say that I came down to earth, do not imagine that I am no longer in heaven. I am here in the body on earth, and at the same time co-enthroned there with the Father in my divine nature.
- Explanation of the Gospel of John; Chrystopher Stade, trans. (Chrysostom Press, 2007), p. 51
It also may be worth pointing out that Christ is no longer speaking to Nicodemus only, but to all Jews. When He begins his discourse, he addresses Nicodemus only - in the singular:
Ἀμὴν ἀμὴν λέγω σοι
Verily, verily, I say unto thee
In the same sentence, though, he shifts to the plural:
οὐ λαμβάνετε
and ye receive not our witness.
The archaic King James English preserves the distinction between the singular and plural forms of "you" ("thee" and "ye", respectively). The distinction is lost in modern translations.