Gregg writes "[Mark] never defines what 'Christian' is and never defines what 'founding' is."
From Did America Have a Christian Founding?:
"By 'orthodox' [Christianity] I mean that they adhered to fundamental Christian doctrines as articulated in the Apostles’ or Nicene Creeds" (164).
In the book's introduction, I consider three possible answers to the question "when was America founded?" and conclude:
"If one is to understand American history, it is important to have a proper appreciation for the nation’s Christian colonial roots. Few serious scholars deny that the early colonists were committed Christians, whose constitutions, laws, and practices reflected the influence of their faith (especially in New England). But the historical debate becomes far more heated with respect to Christianity’s role in the War of Independence and the establishment of the constitutional order under which our nation still operates. For this reason, in this book, I focus almost exclusively on the late eighteenth century." (XXV)
Finally, and once again, the whole point of the chapter "The Myth of the Founders' Deism" is to demolish the common claim that most or many of the founders were deists. Attacking these claims does not require me to prove that they hold any other religious views, and certainly not that they are orthodox Christians. Similarly, I might attack the claim that Barack Obama is a Muslim without having to make any claim regarding his actual faith. This is a simple matter of logic.