I have called protestantism proto-atheism largely because the denial of reason which you find with people like Martin Luther (who famously said that reason was the devil’s whore) and John Calvin (whose doctrine of the total depravity of man makes reason at best unattainable for men) sets it on that course. However, I have recently realized that there is another way in which protestantism is proto-atheism, embedded in what the doctrine of Sola Fide often becomes. (I would like to emphasize that I am talking about protestantism and not protestants, many of whom share little in common with Martin Luther and have a healthy respect for reason.)
According to Peter Kreeft, there is a way in which the doctrine of Sola Fide is in fact compatible with orthodox Christianity (it’s towards the end of that video if you’re looking for it). I have grave doubts that this expansive and non-obvious meaning of Sola Fide was what Martin Luther meant but since he’s dead that’s purely a question between him and God. What is relevant to us, however, is that a great many evangelicals and fundamentalists (and some other protestants) are quite sure that this orthodox interpretation is wrong. They hold that all that is needed to get into heaven is for a person to believe that Jesus is the son of God and died for their sins. Often this takes the form of “accepting Jesus into your life” by saying a prayer where you formally accept Jesus as your personal lord and savior. Often (but not always) it involves some feeling of “knowing that you are saved”. To distinguish this from possible other versions of Faith Alone, I will refer to this version as Belief Alone.
One of the problems which immediately crops up with salvation by belief alone—if you think about it for more than a few seconds—is that after people die and come to meet God face-to-face on the day of judgment, everyone will believe. (As the saying goes, Satan believes.) It is, therefore, not possible that there is anything operative in belief that contributes to or makes up part of the substance of salvation. Worse, since most evangelicals and fundamentalists seem to conceive of heaven and hell as two alternative rooms, one with a party one with far too many sharp things in the hands of unpleasant creatures with odd senses of humor, and in no way think of salvation as any sort of improvement from an imperfect state to a perfect state, belief during life can only be a criteria like how having all six colors of pie piece allows you to attempt to win the game in Trivial Pursuit. It is a purely arbitrary rule.
The only possible purpose of this arbitrary rule—if entry into the infinite party room being only for people wearing the wristband of belief has any purpose at all—is to function as a test of obedience. But, the question must be asked: obedience to whom?
Now this is where the rejection of reason (more formally, fideism) comes up again. If evangelicals and fundamentalists (etc) believed in natural theology, i.e. reason’s ability to approach God, this test of obedience would be very harsh, but it would at least be a test of obedience to God, because a natural man unaided by divine revelation through miracle can still learn of God through reason and thus such belief could—by a great stretch of the imagination—be some sort of test of the individual’s worth. How it can be a meaningful concept for a fallen creature to merit salvation is still something that would need to be explained, but there would at least be some hope for how salvation through belief alone would not be completely self-contradictory (not to mention completely evil).
But when you add in fideism, it is not possible for one to use reason to arrive at the truth. The ticket into the party room thus consists of belief in something one has no reason to believe. Whatever the person proposing this idea may say about asking you to have faith in God, what he is really doing is asking you to have faith in him. Moreover, because—according to him—you cannot know who God is, it cannot be God in whom you believe. You cannot believe in what you cannot know. The end result is that this is nothing other than a demand that you obey the person who is making the demand of you.
As I understand it in the typical case the one making the demand is a person’s parent, but since the demand did not originate with them—they are just passing it on—this really ends up being a demand for absolute fealty to a person’s society. This leads to atheism in two ways.
The first is that this demand is so unreasonable that a reasonable person will utterly reject it. This is why so many of the people who come to the Catholic church from fundamentalism or evangelicalism do so by way of atheism. It is also why modern atheists so often seem like fundamentalists who have simply switched their holy book from the bible which they interpret in light of popular books about it to their high school biology textbook which they interpret in light of popular books about it. (I mean that last part metaphorically, not literally.)
The other way that salvation by belief alone leads to atheism is that it is a form of idolatry. Idolatry is worshiping a created thing in place of the creator, and in this case the created thing is the society. Idolatry is a matter of fealty, i.e. priority, but not necessarily of belief, so this is not simply atheism by name, if it often seems to look like it in practice. What leads it to become avowed atheism is the existence of a another society which the person wishes to be a part of. Sometimes it’s another sub-culture. Often it’s the larger culture of the society in which the fundamentalist/evangelical lives. Whatever it is, this sets up a conflict, and if the other culture wins, a strong rejection of the idol becomes necessary, because it is a jealous idol. Since its official belief in God is part of that idol, it will become rejected when the idol does. The attitude of total fealty to society may not, however, and I believe that this is where we get most of our evangelical atheists from. They have transferred their complete devotion from fundamentalist/evangelical society onto whatever new society they identify with, and will attack believing in God with the same ferocity that they used to attack not believing in God. And their theological knowledge will not have improved from the transition.