What does John 12:36 mean?

Writing in 2 Corinthians 6:2, Paul makes his famous statement that "now is the favorable time; behold, now is the day of salvation." James 4:14 indicates that life is like a vapor that's quickly gone without warning. The book of Proverbs warns that those who are stubborn face a sudden and permanent fate (Proverbs 29:1). Those warnings all share a common theme related to what Jesus is preaching to a crowd in Jerusalem (John 12:20–22). The essential message is: "don't wait."

In a very literal sense, Jesus is telling people that His earthly ministry is coming to a close. Their opportunity to hear and respond to Him is slipping away (John 12:23). Those who reject His words and His message are risking being left in the dark (John 12:35). More generally, He is speaking to anyone who hears the gospel (Luke 7:22–23) or sees the myriad evidence available for truths about God (Psalm 19:1). He's warning mankind that only those who become "sons of light" (1 John 3:1) can avoid being overtaken by darkness (Matthew 25:30). At some point, it will be "too late," and those who have ignored God up to then will have no excuse (Romans 1:18–20).

The end of this verse—which probably would have been better as its own segment—is something of a literal application of those ideas. Jesus leaves the crowd and is "hidden" from them. Whether that simply means He went somewhere private, or was supernaturally covered, Scripture does not say. Either way, this encounter is now closed.

Jesus' statement starting in verse 44 is not tied to a particular time or place; it might be a general summary of His words. That means that what's contained here in verse 36 may be, literally, the last words Jesus "preached" in a public setting prior to His arrest.