"And shewing mercy unto thousands of them that love me, and keep my commandments."
Believing in God is trusting in him and OBEYING HIS COMMANDMENTS.
Those who believe in Jesus Christ: they are saved from hell, because they have received God's mercy and forgiveness of sins. We must believe, to be saved, and to go to heaven.
But the Holy Bible, and God, said that those who sin will go to hell and perish for eternity. This is because sin is the manifestation of disbelief in God.