What is the meaning of the Sign?

Read John 6.

Either attracted or repelled, we are influenced by what we see.  Many in the crowds that followed Jesus were only there waiting to witness the next miracle (v.2).

The Greek word translated miracle (or interchangeably translated sign) refers to a supernatural occurrence with significance.  Jesus never used His power for a show or to enrich Himself.  Each miracle had the intent of proving who He was and confirming His message.  Some got the message; some did not.

Using only a boy's lunch, He fed 5,000 men, plus women and children, with plenty of leftovers for His twelve disciples.  As a result, "When the people saw the sign that he had done, they said, 'This is indeed the Prophet who is come into the world!'" (v.14)  They made a direct connection to Deuteronomy 18:15 where Moses predicted that the LORD would raise up a prophet like him for Israel.  Eager to usher in God's kingdom on earth, they sought to make Jesus their king "by force" (v.15).  But as others have noted, there will be no kingdom without the cross.  His redeeming work of sin must come before His reigning on earth.

What the people truly believed about Jesus as the Messiah surely was mixed among them.  Some followed Him only for what they could get out of it.  Jesus said, "...you are seeking me, not because you saw signs, but because you ate your fill of the loaves." (v.26)  That is when Jesus quickly turned from the miracle to the message.  Food will provide a temporal satisfaction for the body.  Christ offers "food that endures to eternal life" (v.27).  Jesus is that "true bread from heaven" (v.32).  "I am the bread of life" (v.35).  The signs all point to Jesus!

The people asked, "What must we do?"  (vv.28-29)

Jesus responded with point by point clarity.
1. "This is the work of God..." 
No one will experience the forgiveness of sin and entry to heaven by their own self-efforts and good works.  John already made that clear in 1:13.  The Apostle Paul wrote, "For by grace you have been saved through faith.  And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast." (Ephesians 2:8-9)

2. "...that you believe..."
In verse 40, Jesus stated, "For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day."  It is not a belief in a religion, an organization, an icon, or philosophy, but it is a personal trust in a real Person.

3. "...in him whom he has sent."
Later, the Apostle John penned in 1 John 4:9, "In this the love of God was manifest among us, that God sent his only Son into the world, so that we might live through him."

  "For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life." (John 3:16)