"The work of God is this: to believe in the one He has sent." (John 6:29)

Here Jesus is answering a question that arose from his students:
"What must we do to do the works God requires?" (John 6:28)
From Jesus’ statement we know there are two distinct individuals being discussed – the Supreme Being and God's messenger ("the one He has sent").  In other words, there is a sender and the one being sent – two personalities.

Therefore, we can know clearly from Jesus' statement here that he is not and never claimed to be God. He was identifying himself as God's representative – God's messenger. To send someone means to dispatch a messenger.

So why is "believing" in God's messenger so important?

Actually, this is not what Jesus said. 

The word being translated to "believing" is πιστεύω (pisteuō). This word means "have confidence" and "to entrust a thing to one" and "to be intrusted."

In other words, Jesus is not speaking of belief - certainly his students all believed in Jesus. Jesus is speaking of trust: Trusting in Jesus. Thus we have:
"The work of God is this: to trust in the one He has sent."
Why is trust so important? Just consider what we current trust. We might trust in our family, or our job, or our health. But none of these things can truly be trusted because none of them will last. Once our body dies, all of these things will disappear.

Trust in the sense that Jesus speaks goes further - it is putting one's life in the hands of the person we trust. It means to surrender oneself or give of oneself.

This lies at the very crux of spiritual life: The loving relationship between God and His loving servants. To trust the loving servant of God is to honor the one who is being served - the Supreme Being. It means to put oneself under the shelter of the Supreme Being.

A loving relationship

Within the loving relationship between the Supreme Being and His loving servant lies the intent and meaning of all the scriptures. Within this loving relationship lies the importance of Abraham, the instructions of Moses, the songs of David, the devotion of Solomon and the focus of Isaiah. Within this loving relationship we see the true essence of the spiritual world. 

We see the intention of God as He works to bring us back. We see the rationale for all the suffering and pain upon the temporary physical bodies of the illusory material world. We see the reason for prayer; for songs of praise; for temples and offerings. We see the reason for beauty, kindness and compassion. 

We see the reason we are away from God now. And finally, we see the essence of the relationship between Jesus and God, why Jesus can say "I and the Father are one," and the reason why Jesus sacrificed his physical body in God’s service.

All of this can be seen within the relationship that exists between God and His loving servant. The loving servant is the embodiment of this relationship, simply because the loving servant works to please God, and the desire to please God is the symptom of love for God – the first and foremost commandment.

This relationship is so important to God that even God Almighty treasures this relationship, and treasures the one who shares it with Him.

Trust in what?

Furthermore, to "trust" in Jesus means to trust in his instructions, because those instructions are ultimately coming from the Supreme Being. And by trusting in the instructions of God's messenger, we can learn to enter the relationship of love that exists between the Supreme Being and His loving servant. To enter into such a relationship of loving service with God is to satisfy the primary instruction of Jesus, Moses, and all other bonafide representatives of the Supreme Being:
“ ‘Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment.” (Matt. 22:37-38)