There is a sense in which D'Souza is right. Yes, Christians from a variety of traditions can agree on much, and often work together in advancing the common good in a variety of causes both inside and outside their respective communities. And he is indeed correct that Christians, as well as other theists, should make a winsome and intelligent case against the philosophical materialism on which the most pernicious affects of secularism rely. D'Souza has made important contributions to advancing such a case, and even has been wisely circumspect in distancing himself, though respectfully, from those Christians who believe that intelligent design should play an integral role in the project of the Christian philosophy of nature. (My own pilgrimage on this matter may be found on the BioLogos website).
But there is a sense in which D'Souza is wrong. Although it is certainly true that the Apostle's Creed and Lewis' Mere Christianity reflect the barest one may believe in order to count as a "Christian," it does not follow that they are the basis by which one may define what counts as a "mere squabble." After all, if, let's say, a Unitarian were to tell D'Souza that he considers himself a Christian but cannot accept either the Creed or Lewis's "mere Christianity," D'Souza would say that the Unitarian is not a Christian based on the Creed/Lewis standard D'Souza embraces. But what if the Unitarian were to respond, "A lot of times, Christians spend a lot of time in intramural type debates and squabbles. Are you a Trinitarian or Unitarian; if you are a Unitarian, what type are: are you a humanist or theist; what position do you take on the resurrection of Christ?" Why is D'Souza's "mere Christianity" not just another position in a different squabble, at least according to the Unitarian?
The "Creed/Lewis" standard is something that evangelicals, Roman Catholics, Anglicans (like Lewis!) and capital O "Orthodox" agree forms a lowest common denominator of "mere Christianity." Anything that falls outside of that LCD (Jehovah's Witnessism, Mormonism, theological unitarianism) is not "Christian." There is a big gulf between that standard and "anything that calls itself Christian is Christian."
The American Founding, in a political theological sense, may be "Christian" according to the later, but not the former. Jefferson, J. Adams and Franklin clearly rejected this kind of "mere Christianity" (most folks don't know Adams rejected "mere Christianity" more clearly than Franklin did) and Washington, Madison, G. Morris, and many others are not provably "mere Christians."
I found the President [James Madison] more free and open than I expected, starting subjects of conversation and making remarks that sometimes savored of humor and levity. He sometimes laughed, and I was glad to hear it ; but his face was always grave. He talked of religious sects and parties, and was curious to know how the cause of liberal Christianity stood with us, and if the Athanasian creed was well received by our Episcopalians. He pretty distinctly intimated to me his own regard for the Unitarian doctrines.— TICKNOR, GEORGE, 1815, Letter to his Father, Jan. 21 ; Life, Letters and Journals, vol. I, p. 30.
That Washington was a professing Christian is evident from his regular attendance in our church; but, Sir, I cannot consider any man as a real Christian who uniformly disregards an ordinance so solemnly enjoined by the divine Author of our holy religion, and considered as a channel of divine grace. This, Sir, is all that I think it proper to state on paper. In a conversation, more latitude being allowed, more light might, perhaps, be thrown upon it. I trust, however, Sir, you will not introduce my name in print.
I am, Sir,