[drummojg]

It's, for lack of a better term, "High English." It is meant to sound grand, and thus the grander old style was used. Which is entirely appropriate; IIRC the original Hebrew uses grander language for poetic passages & the words of the LORD.

[hajile]

Thou = singular second person

You = plural second person

It uses them because there is an actual difference. In modern English, you cannot tell if "you" is referring to one individual or a group without examining the surrounding context and adding your own judgement (a translator looking at the original text will have much better judgement).

[shaftoe444]

Even in Shakespeare the use of them is pretty inconsistent grammatically but makes perfect sense as a stylistic choice.