Jesus was responding to a group of people who asked Jesus what
they could do to do the works of GOD, not what GOD could do for them.
Do not work for food that spoils, but for food that endures to eternal life, which the Son of Man will give you. For on him God the Father has placed his seal of approval.
Then they asked him, “What must we do to do the works God requires?
Jesus answered, “The work of God is this: to believe in the one he has sent.” John 6:27-29
It is men who believe, not GOD, so it is man's work.