Sed longe omnium indignissimum se putare, quod Ecclesiastas dicas expostulare

[limetrees]

Longish, but trickiest bits are in bold, so if anyone wants to read and comment, I’d be grateful.

Negant quoque magis te potuisse causam agere Remonstrantium, uno hoc excepto, quod non aperte Zelotarum opinionem erroneam dicas. Sed longe omnium indignissimum se [should this be “te”?] putare, quod Ecclesiastas dicas expostulare, quod non intersint conventibus publicis; Ἀρχαιρεσίαν eos [ecclesiatas] multis in locis obliquas per artes in se trahere, et ubi invaluerunt vindicare etiam sibi audere; Δημαγωγίαν saepe ad negotia civilia extendere; Flandriam turbulentis suis concionibus perdidisse; tempore Licestrii hoc egisse, ut Ordines redderent contemptissimos; Ordines necesse habuisse, eos publicitus monere, ut perdenda Hollandia absisterent; periculosissimo tempore Ordinum detrectasse imperium, ivisseque oblatum externis.

They also deny that you could support the cause of the Remonstrants any more, with this exception, that you do not openly say that opinion of the Zealots is wrong. But [they say] that they think [it] by far the most unworthy of all, what you say you find fault with the preachers for, that they do not participate in the public assemblies; that they draw the election of magistrates to themselves in many places by oblique means, and when they are strong they dare to claim it for themselves; they extend [their] leadership of the people to civil business; that they lost Flanders with their turbulent assemblies; that they did this in the time of Leicester [Dudley, 1st Earl of Leicester], so that they made the States abject; that the States found it necessary to warn them publicly that they cease from ruining Holland; that they damaged the rule of the States at the most dangerous time, and aimed to offer [that rule??] to outsiders.

Text from here.
http://grotius.huygens.knaw.nl/letters/0299

 

[Pacifica]

They also deny that you could support the cause of the Remonstrants any more

Maybe it's just me, but couldn't this be interpreted as "they deny that you could support their cause any longer"? Or maybe the following context is enough to exclude this interpretation.

Sed longe omnium indignissimum se [should this be “te”?] putare

No, se is the subject of putare ("they say that they think"), and indignissimum refers to the following fact introduced by quod, not to "you". Your translation here is right:

But [they say] that they think [it] by far the most unworthy of all

quod Ecclesiastas dicas expostulare, quod non intersint conventibus publicis
what you say you find fault with the preachers for, that they do not participate in the public assemblies

Ecclesiastas is the subject of expostulare. "... that you say that the preachers complain of the fact that they do not..."

et ubi invaluerunt vindicare etiam sibi audere
and when they are strong they dare to claim it for themselves

You forgot to translate etiam, and perhaps "grow powerful" would be better than "are strong". I would perhaps formulate it like this: "and, when they grow powerful, even dare to claim it for themsleves" or "and even dare to claim it for themselves when they grow powerful".

Δημαγωγίαν saepe ad negotia civilia extendere
they extend [their] leadership of the people to civil business

You've forgotten to translate saepe.

tempore Licestrii hoc egisse, ut Ordines redderent contemptissimos
that they did this in the time of Leicester [Dudley, 1st Earl of Leicester], so that they made the States abject

Hoc is only anticipating the ut clause, it represents it and not another "this". "... that in the time of Leicester they did so that they made..." or perhaps "... that what they did in the time of Leicester was that they made..."

I didn't know that ordines had ever been used with the meaning "states", but I suppose you translated it so knowing the historical context and the particular use of this word in it?

Ordines necesse habuisse
that the States found it necessary

Maybe just "had to/were forced to".

detrectasse
damaged

I see two definitions for detrectare: "to refuse/reject" and "depreciate/detract from/disparage". I'm not sure which one is meant here.

ivisseque oblatum externis
and aimed to offer [that rule??] to outsiders..

"Went to offer it to outsiders". I suppose imperium is indeed still implied here.

 

[limetrees]

optime!

gratias magnas tibi, illustrissima doctissimaque.