"If we give any credence to the architects of the structure, it is clear that they believed both design and Designer of our constitutional republic were 'self-evident.' They believed that the bedrock upon which rested its posterity is found in – and only in – the pages of Holy Scripture."
I've read the Constitution, notes on the Constitutional Convention and the Federalist Papers and they DO NOT ASSERT THIS. Certainly the Bible was *a* source of inspiration for the Founding Fathers (and even the Donald S. Lutz, et al., study shows the Bible was at its LEAST import when the Federalists WROTE and RATIFIED the Constitution). There were other sources of inspiration as well, in particular the noble paganism of republican Rome. The Founders adopted those surnames NOT Hebraic or biblical ones.
Likewise YOU may be able to connect separation of powers to the Bible. But the Founders DID NOT cite the Bible for the proposition, but rather, as you note, Montesquieu.