Identification: This Latin passage is the opening of Book XIX (Lib. XIX, cap. 1) of Augustine's De Civitate Dei (The City of God). The phrase beginning “Quoniam de civitatis utriusque, terrenae scilicet et caelestis, debitis finibus…” is the first sentence of Book 19.
Where it sits in the whole work: City of God is made of 22 books. Book XIX is therefore late in the work (19th of 22). So, relative to the very first page of the entire City of God it is well into the manuscript — roughly in the final quarter of the work. However, the sentence is at the start of Book 19 itself, so if you mean “how close to the first page of Book 19,” it is the first page of that book.
Short translation (one sentence): “Since I seem henceforth to have to discuss the proper limits of each city, namely the earthly and the heavenly, I must first explain how far the nature of this work allows me to complete it, and the arguments of mortals by which they have tried to make a happiness for themselves in the miseries of this life, so that our hope may differ from their vain things only insofar as God has given us it; and the thing itself, that is true beatitude which he will give, may be made clear not only by divine authority but also by reason applied as far as we can, on account of unbelievers.”
On 14th‑century manuscripts: Medieval manuscripts vary in layout, quiring, and foliation. In a 14th‑century copy the start of Book XIX will normally appear at the beginning of a folio or column and may be marked by a rubric or decorated initial. But exactly how many folios into the codex that is depends on that particular manuscript’s arrangement (whether it contains only De Civitate Dei or additional works, how large the hand is, etc.).
If you want a precise folio location: give the manuscript’s shelfmark (library identifier) or upload a photo of the folio; I can then tell you whether the passage you quoted is on the first page of Book XIX in that specific codex and how many folios it lies from the start of the volume.
References you can check: look up Augustine, De Civitate Dei, Book XIX, chapter 1 in any standard edition or translation (e.g. Henry Bettenson, trans., Penguin; or the Latin in a critical edition or Patrologia Latina). The line you gave matches the standard Latin text for the opening of Book XIX.