What does John 3:32 mean?

Jesus' message is based on His own first-hand knowledge. This is why He is the only valid source of spiritual truth. Only Jesus has come from Heaven (John 1:1; 3:13). Only those who believe His message can be saved (John 3:18). Those who won't listen to the common, ordinary things Jesus teaches have no hope of understanding the spiritual things (John 3:12). The Greek word referring to giving testimony is martyreō, which means "to report," or "to affirm," in the same sense that a person's statement in court is called their "testimony." This is why those who die specifically for their testimony of faith are called "martyrs." The apostle John uses forms of this term thirty times in his writing (e.g., John 1:7; 5:36; 19:35; 21:24; 1 John 5:11; Revelation 1:9; 20:4).

A claim to personal knowledge is one reason Jesus often uses the phrase, "amen, amen." The word amen has been preserved from translations through Greek, Hebrew, and into English. This is usually translated as "verily, verily," or "I tell you the truth," or "I assure you." Starting a statement with this phrase is a way of claiming personal knowledge: not something learned and repeated, but an original, intimately known fact.

Sadly, most people will not accept the message of Christ (Matthew 7:14). The end of this verse uses a common technique of exaggeration. We do the same in modern speech: when one in a thousand people accept something, we might say, "no one thinks that's true." Of course, in the most literal sense, someone does, but the point is clear. Here, in the context of the very next verse, we see what John means. Very few accept the Light (John 1:9–10), because it forces them to confront their sins (John 3:19).