Thesis 1: holding the principle of sufficient reason (PSR), which states that “there is a sufficient reason or adequate necessary objective explanation for the being of whatever is and for all attributes of any being” , or alternatively that "for every fact F, there must be an explanation why F is the case" , either is equivalent to holding the following two notions, or presupposes holding them:
R1. Subsistent, Ultimate Reality is logos, i.e. reason. 
R2. Human reason is created in the image of the uncreated Logos.
Of course, the two notions above are held in conjunction with this:
R3. Created reality was created according to reason. 
which is clearly consistent with the observable fact that the universe works causally according to mathematically expressible laws.
A panentheist can hold these notions by replacing "created" with "created/emanated" in R2 and R3. I mention this possibility to allow for panentheism as alternative "bootstrap" position or entry point, which could then, by reasoning on the PSR, be corrected into classical theism. Which makes sense considering the numbers of Taoists, Mahayana Buddhists, Hinduists and Sikhs.
Noting that Prof. Feser stated that "to see the world as intelligible or rational through and through is implicitly to be a (classical) theist" , the basis for a possible demonstration of Thesis 1 is: why else should we assume that reality is ultimately rational? This can be perceived more clearly if we examine the alternative interpretation of observed facts that a materialistic evolutionist could propose instead of that based on the PSR, either in its traditional form or in the form of the Rx above (which is equivalent to the traditional form if this thesis is correct):
M1. Brute fact: the universe exists and works causally according to laws expressed mathematically.
M2. The rationality of our mind, i.e. the agreement between the way it works and the way the universe works, was selected by evolution. Because, on seeing the branches of a bush moving in windless weather:
- the walking-talking apes who thought the movement had a cause got ready to fight or flee, survived, and passed on their genes.
- the walking-talking apes who thought the movement did not have a cause did nothing, and were killed by a rival tribe or an animal.
M3. At some point, some of the walking-talking apes that were so evolutionary successful because, among other things, the way their mind worked conformed to the way the universe worked, got the big picture the other way round, and thought that it was the way the universe worked which conformed to the way their mind worked. That would have had no practical consequence, but some of them went even further, and claimed that their mind was able to explain reliably not only the way the universe worked, but even why there was a universe! And the apes called that statement the Principle of Sufficient Reason, and they rejoyced greatly.
"Hey, walking-talking ape, who do you think you are?"
This thesis originated from my realization on Sep. 2014 that the modal cosmological argument, or argument from contingency, is just the principle of efficient causality (PC), and that the PC in turn is based on just the Principle of Sufficient Reason, the latter statement being confirmed by Prof. Feser on Nov. 02, 2014 . Thus, the teaching in First Vatican Council's Constitution "Dei Filius" "that God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural light of human reason "for his invisible attributes, ever since the creation of the world, have been clearly perceived, being understood through the things that have been made."" (ch. 2 "On Revelation", which in turn quotes (Rom 1:20)) presupposes the implicit condition "if the person in question assumes that reality is ultimately intelligible/explainable by human reason".
Now, whereas philosophy is about the rational explanation of reality, the assumption that reality is ultimately rationally explainable is meta-philosophical, i.e. holding the PSR is a meta-philosophical choice. Thus, the issue of theism vs atheism is not really philosophical but meta-philosophical, as the latter position is based on the assumption that reality is not ultimately rationally intelligible/explainable. Which is exactly David Hume's position as summarized by Texas A&M University Prof. of Philosophy Stephen H. Daniel :
"The argument assumes that the world's existence can be explained rationally by appeal to God as its cause; but why should we think that the world's existence is rationally explainable?"
Or Bertrand Russell's position in his famous debate with Fr. F. C. Copleston :
"R: The whole concept of cause is one we derive from our observation of particular things; I see no reason whatsoever to suppose that the total has any cause whatsoever.
R: what I'm saying is that the concept of cause is not applicable to the total.
R: I should say that the universe is just there, and that's all.
R: for my part, I do think the notion of the world having an explanation is a mistake. I don't see why one should expect it to have,
C: But your general point then, Lord Russell, is that it's illegitimate even to ask the question of the cause of the world?
R: Yes, that's my position."
Or Prof. Sean Carroll's position, as stated in 2007 :
"There is a chain of explanations concerning things that happen in the universe, which ultimately reaches to the fundamental laws of nature and stops. ... There is a strong temptation to approach the universe with a demand that it make sense of itself and of our lives, rather than simply accepting it for what it is."
and again in 2012 :
"It’s okay to admit that a chain of explanations might end somewhere, and that somewhere might be with the universe and the laws it obeys, and the only further explanation might be “that’s just the way it is.” ... I could be wrong about that, but an insistence that “the universe must explain itself” or some such thing seems like a completely unsupportable a priori assumption."
"Now if PSR is false, then the principle of causality is threatened as well, since if things are ultimately unintelligible, there is no reason to think that a potency might not be actualized even though there is nothing actual to actualize it and thus that something contingent, like the universe, might just be without any cause at all. But then it would not be possible to argue from the world to God as cause of the world."
 Bernard Wuellner, Dictionary of Scholastic Philosophy, p. 15.
 Melamed, Yitzhak and Lin, Martin, "Principle of Sufficient Reason", The Stanford Encyclopedia of Philosophy (Spring 2015 Edition), Edward N. Zalta (ed.), URL =
 In "Subsistent, Ultimate Reality is logos", "logos" is meant as divine attribute common to the three divine Persons, not the Logos as divine Person, the Son. Just as in "God is love" (1 Jn 4:8,16), "love" is meant as divine attribute common to the three divine Persons, not Love as divine Person, the Holy Spirit.
 In line with "All things came into being through Him" (Jn 1:3) and "in Him all things were created" (Col 1:16), where "Him" is the Logos as divine Person, the Son.
 Test Questions for Phil 251: Intro. to Philosophy, Philosophy of Religion, option 101.B
(where options 99 and 101 should say "cosmological", not "teleological", argument).