It’s not necessary to have faith in things that are consistent with reason or, therefore, to doubt things that are consistent with reason. Faith is necessary in order to believe things that are inconsistent with reason, that have no logical explanation, or that are actually inconsistent with observable facts. In such cases doubt is the logical reaction, not faith.
Doubt is not even un-Christian. The most famous Christian doubter is the disciple Thomas. He was asked to accept something that defied reason, based on the testimony of his fellows. Had they told him that they went to see Jesus’ body laid in state and that his countenance was peaceful, Thomas would likely had taken their word for it. That would be consistent with what one could reasonably expect based on previous experience. But Thomas wasn’t asked to accept something that was consistent with reason. He was asked to accept that Jesus had come back from the dead and entered a locked room without opening the door. Extraordinary claims demand extraordinary evidence, and Thomas refused to accept the claims until he had personally witnessed Jesus’ return. Thomas was a scientist. He refused to accept things on faith alone. He demanded evidence, personal observation. For the most part, nobody faults him for this.
Religious people often argue (when trying to discredit science) that science is itself just another religion, that in order to accept scientific truth one has to have faith in things that they have not personally experienced. This is a false equivalence. Yes, scientists have faith in the veracity of some things that they have not themselves verified to be true. Few scientists have personally measured the charge on the electron, but few scientists doubt that it is 1.602 x 10-19 Coulombs. (I actually got to measure this for myself in a sophomore physics lab, and it was one of the most exciting labs of my undergraduate career.) Accepting the value of the electronic charge is a lot different from accepting the existence of an all-powerful god who regularly intervenes in human affairs. For starters, there is an experiment that one can carry out in order to independently verify the electronic charge. The apparatus is described in detail in the scientific literature. It’s not all that complicated, well within the reach of the average person. The calculations are complicated, but again they are well documented and fully explained in many sources. A lot of people have done the experiment and verified the result. It’s settled science. It’s not, therefore, necessary for every individual to do the experiment themselves to believe the result. It’s not inconsistent with reason. There’s no good reason to doubt the result, which has been used to do countless other experiments and obtain the expected results.
By contrast, nobody has ever designed an experiment that can verify the existence of an all-powerful god who regularly intervenes in human affairs, certainly not one that satisfies the requirement of being replicable. There’s no evidence for such a claim, plenty of evidence to suggest it’s not true, and therefore plenty of reason to doubt its truth.
Scientists do not have faith in the individual findings of science. For those we have evidence, rendering faith unnecessary, and doubt pointless. Only a fool continues to doubt once confronted with solid evidence. Thomas put aside his doubt when Jesus appeared to him and invited him to make his own observations. Extraordinary claims require extraordinary evidence, but if that evidence is presented then the rational individual will put aside their doubts.
One could argue that scientists have faith in the scientific method, but I’m not sure faith is the right word for that. We have confidence in the scientific method. It has proven very good at debunking false claims and correcting poorly executed experiments. It leads to the ability to predict the results of other experiments. It is self-correcting. Those who make false claims are found out when their conclusions cannot be repeated. It goes against reason (though some will believe it, on faith) that there is a massive conspiracy involving hundreds of thousands of scientists bent on obscuring the truth about global warming, or evolution, or the Biblical creation story. Humans just aren’t that good at cooperation. Somebody would screw up and let the cat out of the bag. It’s much more reasonable to doubt that such a conspiracy exists than to doubt the scientific conclusions themselves. Extraordinary claims require extraordinary evidence, and nobody has presented evidence for global-scale scientific conspiracies.
Personally, I doubt very much that there is an interventionist god as described in the scriptures. In fact, I’m quite certain of the opposite, so certain that I don’t think of my lack of belief in the Christian caretaker god as doubt at all. Doubt is essential to my lack of belief though. For instance, I doubt very much that if there were a god powerful enough to create the universe, and to breathe life into lifeless forms, that they would behave anything like the god described in the Bible. The god of the Bible is angry, petulant, vengeful, insecure, and just a generally unpleasant character. Richard Dawkins gives a more complete list in his “The God Delusion”, and Dan Barker provides exhaustive evidence from scripture for all of the negative characteristics of the Christian god in his “God: The Most Unpleasant Character in all Fiction.” The petty jealousy and obsession with other gods exhibited by the god of the Bible is much more consistent with someone who has little power and is bitter about it than with a being secure in their power. It’s entirely believable that the rabbis and priests trying, often unsuccessfully, to use religion to control the population would exhibit the characteristics they ascribe to their god, but not believable at all that their god should exhibit those characteristics. It seems like projection to me.
I fall back on the ancient Greek philosopher Epicurus, an early doubter, for the most convincing argument against faith that I’ve encountered. The logical argument from evil is as follows: If an omnipotent, omnibenevolent and omniscient god exists, then evil does not. There is evil in the world. Therefore, an omnipotent, omnibenevolent and omniscient god does not exist. Epicurus is often quoted in memes with the following: “Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God?” This argument does nothing for the proposition that there is a god, of some sort, a creator god perhaps. The existence of a creator god is an un-testable hypothesis, not worthy of either faith or doubt. Certainly there exists the possibility of a god that is not omnipotent, and there exists the possibility that god is malevolent, but in neither case would they be worthy of worship. The Christian God, however, is supposedly both omnipotent and benevolent. It is not reasonable to believe that He would rain down misfortune on Job in order to settle a bet with Satan. It’s not reasonable to believe that he turn people into pillars of salt for the “crime” of looking over their shoulder at the ruins of their home city. Those aren’t the actions of a benevolent god secure in his omnipotence. To believe in that god requires faith (which I began by defining as the belief in things that are inconsistent with reason, that have no logical explanation, or that are actually inconsistent with observable facts), and I don’t have faith. I have doubt. Reason requires it.