He shall give his angels charge over thee, to keep thee.
Why is charge uncountable here?
God gave Joshua a charge.
In this case, it is countable.
I can't find authoritative sources.
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Various senses of charge are given at the Macmillan entry. Look at the tags (countable; countable/uncountable; singular) added to each.
However, three 'phrases' [I'd say idioms] containing the noun 'charge' are also given:
in charge [of]
in someone's charge
take charge [of]
None of these idiomatic usages inflects.
The old-fashioned 'give them charge over' is a relative of 'in someone's charge' and similarly does not inflect. A similar idiom that is common today is 'give them control over', where 'control' does not inflect.