I assume that quantulum abest is clear: “How little is missing.” Actually, it is a little unclear, because I would personally have expected deest here, but they are certainly not that different.
What may be more confusing is quo minus, and the simple answer is that it is a fixed expression, often written as one word – quominus – which is usually found after expressions of hindering and preventing, like prohibere, continere, etc., and introduces a subordinate clause in the subjunctive, meaning “so that not,” often simply translated as “from.” For example: deterrere aliquem, quo minus = “to deter someone from”; or: vix se continuit, quominus in in cachinnos erumperet = “he barely kept himself from breaking into laughter.”
The underlying more general construction is a final relative clause starting with the ablative neutral relative pronoun quo and a comparative. Allen & Greenough give the following example:
comprimere eorum audaciam, quo facilius ceterorum animi frangerentur
to repress their audacity, so that [= by which repression] the spirit of the others might be broken more easily
Note that the ablative is not one of comparison, nor (I believe) one of degree of difference. A & G explain the preceding example with the English “by which the more easily,” but I dislike this “the” because it hints at an ablative of degree. In fact this is an ablative of means, and the antecedent is typically the whole main clause and not a specific word in it.
Transferring this understanding of the construction to quo minus we also need to understand that minus can often essentially mean “not,” because in these sentences the subordinate clause is usually not something that you can do more or less (or put differently, doing it less means not doing it at all). Take the following example:
Infirmitas me tenuit, quominus ad ludos venirem.
Sickness held me back, through which holding back I went less to the games = though which holding back I did not go to the games = held me back from going to the games.
While I believe this is a pretty sensible explanation, it is in my opinion much more helpful to remember that this is a fixed expression, and not think too deeply about it.
Returning to your example, the “expression of hindering” is also somewhat obscure, as abesse is not really a verb like deterrere or prohibere; but, in fact, it is of course that because a little bit is still missing, something does not happen (and is thereby prevented) that would happen if that little bit were also present (si adesset). So we get:
How little is missing from the whole Christian world being divided etc.
= How little would have to happen (or would be needed) for the whole Christian world to be divided etc.
