Believe in Him whom He sent (John 6)

“I am the bread of life. He who comes to Me shall never hunger, and he who believes in Me shall never thirst. But I said to you that you have seen Me and yet do not believe. All that the Father gives Me will come to Me, and the one who comes to Me I will by no means cast out.

For I have come down from heaven, not to do My own will, but the will of Him who sent Me.

This is the will of the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day.

And this is the will of Him who sent Me, that everyone who sees the Son and believes in Him may have everlasting life; and I will raise him up at the last day.

”Then they said to Him, “What shall we do, that we may work the works of God?” Jesus answered and said to them, “This is the work of God, that you believe in Him whom He sent.”

This chapter starts with our Lord feeding the 5 thousand and many are drawn to Him. Then He walks on the sea. 
Then He is rejected and many of the disciples following leave Him. 
They could not continue to believe in Him.

We start by believing in Him who was sent for us, as we journey through life trusting in Him we grow into many and different works in Him.

And truly Jesus did many other signs in the presence of His disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name.