Presunção in this case is not a presumption, conjecture or supposition. It has a pejorative meaning and stands for someone who is imodest, arrogant or vain.
Therefore, adequate synonyms could be arrogance, chutzpah, aloofness...

Água benta (holy wsater) is used in the idiom to contrast, at the same time being similar in some aspect to the arrogance.

The idiom means the person can exaggerate in his vanity, thinking too much about himself, without causing any harm. Likewise, he can use or drink as much holy water as he wants and that will do him no harm.

So literally, the idiom would be something like "arrogance and holy water" (meaning absolutely nothing in English).
And what you need is to say the guy is too arrogant but harmless.

So let's use our creativity:

1) Inflates his ego as someone who blow into a baloon with a hole
2) Full of himself making a fool of himself
3) Full of himself yet fool of shit
4) Too big for his boots but too small for his hat (my choice)


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Note added at 57 mins (2014-04-17 22:22:35 GMT)
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1) ** blowS
3) ** FULL of shit