Acts 16:31 (KJV)
31 And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
When you believe something is true in your mind, it will change the way you think; when you believe something is true in your heart, it will change the way you act.
Romans 10:9-10 (KJV)
9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.