The Gospel....

Defining Belief and Unbelief
Let me begin by defining belief and unbelief. Jesus said in John 6:35, "I am the bread of life; he who comes to me shall not hunger, and he who believes in me shall never thirst."
I take it, then, that unbelief in Jesus (NOT believing in Jesus) is a turning away from Jesus in order to seek satisfaction in other things. And BELIEF in Jesus is coming to Jesus for the satisfaction of our needs and our longings.
Belief is not mainly an agreement with facts in the head; it is mainly an appetite in the heart which fastens on Jesus for satisfaction. "He who comes to me shall not hunger and he who believes in me shall never thirst!"
Therefore eternal life is not given to people who merely think that Jesus is the Son of God. It is given to people who drink from Jesus as the Son of God. "The water that I shall give him shall become in him a spring of water welling up to eternal life" (John 4:14). He is the bread of life for those who feed on him—who get their nourishment and satisfaction from him. That is what it means to believe on the only begotten Son of God and be saved

from DesiringGod.org