Believe on the Lord Jesus Christ and you will be saved. It's in the Bible, Romans 10:9, NIV. "If you confess with your mouth, 'Jesus is Lord, and believe in your heart that God raised Him from the dead, you will be saved."
God says we're right with Him if we believe. It's in the Bible, Genesis 15:6, NIV. "Abram believed the Lord, and He credited it to him as righteousness."
What you do can't be separated from what you believe. It's in the Bible, Deuteronomy 27:10, NIV. "Obey the Lord your God and follow His commands and decrees that I give you today."
True "believing" in the Bible is a relationship with God, not just an opinion. It's in the Bible, John 14:15, NIV "If you love Me, you will obey what I command."
What kind of believing does God reject? It is not enough to believe God exists. It's in the Bible, James 2:19, TLB. "Are there still some among you who hold that 'only believing is enough? Believing in one God? Well, remember that the demons believe this too—so strongly that they tremble in terror!"