The Old Latin version had fundabo instead of aedificabo in Matthew 16:18. There have been conflicts over that word... but this is one of those places where the liturgy preserves an older text that the Vulgate.
I think it's an allusion to Matthew 7:24-27, not a quotation.
I think that's right. The Antiphonale Monasticum III, 318 gives no scriptural reference, which it only does when the text is not a direct quotation from Holy Scripture. Laus Divina provides Mt. 7:25 as reference.
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