"So the people of Nineveh believed God . . . "
Somewhat surprisingly, statements like this one (above), are very infrequent in the Bible. By believing God's prophet (Jonah), the pagan people of Nineveh "believed God." To add to the irony, the singular incident in the Word of God where the prophet sent to proclaim the Lord's message actually did *not* want it to be believed, is when it *is* believed. (Remember: Jonah wanted the Ninevites destroyed, not saved.)
What does it mean for a churchman to "believe God" today? It means that we believe the message of God's ministers: the gospel message (in particular). When we do, we are believing in the Son of God, whose blood cleanses us from our sins; and whose grace binds us to Him, in His church.
[Puritan quote of the day: "The human nature of Christ is yet in being. He still continues, and will continue to all eternity, to be both God and man." --Jonathan Edwards, in his sermon, "True Saints are Present with the Lord"]