In 1 Kings 18, Elijah conducts an experiment to determine whether Yahweh or Baal is the true God:
"And Elijah came to all the people, and said, 'How long will you falter between two opinions? If the Lord is God, follow Him; but if Baal, follow him. ...Therefore let them give us two bulls; and let them choose one bull for themselves, cut it in pieces, and lay it on the wood, but put no fire under it; and I will prepare the other bull, and lay it on the wood, but put no fire under it. Then you call on the name of your gods, and I will call on the name of the Lord; and the God who answers by fire, He is God.' " (1 Kings 18:20-24)When nothing happens to Baal's altar, Elijah mocks Baal for his inaction. Then Yahweh sends down fire from heaven to consume the offering to affirm his status as supreme being. Finally, Baal's followers are captured and executed. I just love happy endings, don't you?
Then there are the tests that Gideon conducts to ensure that God will fight for them in Judges 6–7:
"So Gideon said to God, 'If You will save Israel by my hand as You have said— look, I shall put a fleece of wool on the threshing floor; if there is dew on the fleece only, and it is dry on all the ground, then I shall know that You will save Israel by my hand, as You have said.' And it was so. When he rose early the next morning and squeezed the fleece together, he wrung the dew out of the fleece, a bowlful of water. Then Gideon said to God, 'Do not be angry with me, but let me speak just once more: Let me test, I pray, just once more with the fleece; let it now be dry only on the fleece, but on all the ground let there be dew.' And God did so that night." (Judges 6:36-40)I have to admire Gideon's diligence here. One test isn't enough to convince him that God is on his side; he requires two. The evidence is weak by modern standards, but in that superstition-addled culture this would have been a rare moment of clarity. Gideon goes on to defeat the Midianites and execute its two princes. (I'm beginning to sense a pattern with the endings of these Old Testament stories.)
Finally we have the standard for prophecy that God offers in Deuteronomy 18. How do we determine who is a real prophet and who is a fraud?:
"But the prophet who presumes to speak a word in My name, which I have not commanded him to speak, or who speaks in the name of other gods, that prophet shall die. And if you say in your heart, 'How shall we know the word which the Lord has not spoken?'— when a prophet speaks in the name of the Lord, if the thing does not happen or come to pass, that is the thing which the Lord has not spoken; the prophet has spoken it presumptuously; you shall not be afraid of him." (Deuteronomy 18:20-22)The standard that God gives via Moses is simple and 100% evidence-based. If the prophecy doesn't come true, the prophet is not of God—no exceptions. The false prophet must then be executed. (That's three for three!)
So what have we learned here? The message of these passages is pretty clear, though it may not be one that the writers originally intended: when it comes to god-related claims, what really matters is the evidence. If you call on your god to do something and nothing happens, that god is a false one worthy only of mockery. If you think your god wants you to do something, ask him to communicate in a substantive, verifiable way to confirm it. And if someone claims to be a divine prophet, they had better have a perfect track record of successful predictions to show for it.
How, then, could anyone fault atheists for their unbelief? If we test God and his representatives and they fail to measure up, our response is exactly the one that the Bible itself endorses—well, minus the capital punishment. It's perfectly acceptable to ask for an impressive, objective, physical demonstration of God's power. It's perfectly reasonable not to put stock in the prophets of the Bible when their prophecies fail. And it's perfectly fine to disbelieve in God (and based on Elijah's response, even mock him) since he makes no demonstrable impact on the world.
Now, one can also point to countless instances where the Bible takes exactly the opposite view: that faith without evidence is a virtue, and skepticism toward extraordinary claims is a vice. One verse decrees that we should completely trust God over our own understanding. Another even specifically says not to test God. It's not surprising that one can find both stances; all this demonstrates is that the Bible is not a particularly consistent or unified book. What I've shown here is that the Bible does endorse a skeptical viewpoint in a few isolated cases—and that's enough to show that God fails to meet his own standard of evidence.