But meanwhile, for as long as your Reverend Paternity will be determined to tell these shameless lies, others will be permitted, on behalf of his English Majesty, to throw back into your Paternity’s shitty mouth, truly the shit-­pool of all shit, all the muck and shit which your damnable rottenness has vomited up, and to empty out all the sewers and privies onto your crown divested of the dignity of the priestly crown, against which no less than against the kingly crown you have determined to play the buffoon. […] For I am ashamed even of this necessity, that while I clean out the fellow’s shit-­filled mouth I see my own fingers covered with shit. […] [Luther] had got the better of cacodaemons in impiety, in order to adorn his sect with fitting emblems, surpassed magpies in chatter, pimps in wickedness, prostitutes in obscenity, all buffoons in buffoonery.

Interea vero, quam diu constabit reverendam paternitatem vestram, tam impudenter ista mentiri: licebit aliis pro maiestate anglica, lutum et stercus omne, quod vestra putredo damnabilis egessit, in vestræ paternitatis os stercoreum, et stercorum omnium, vere sterquilinium regerere: et in coronam vestram, sacerdotalis coronæ dignitate exauctoratam, in quam non minus ac regiam scurrari decrevistis, omnes cloacas et latrinas effundere. […] Etenim me necessitatis etiam huius pudet: quod dum os hominis merdosum detergo: digitos mihi concacatos video. […] quum cacodæmones impietate vicisset, ut dignis emblemmatis ornaret suam sectam, picas garrulitate, lenones improbitate, prostibula obscænitate, scurras omnes scurrilitate superarit.

Luther's lavatory thrills experts

Martin Luther was candid about his constipation

Archaeologists in Germany say they may have found a lavatory where Martin Luther launched the Reformation of the Christian church in the 16th Century.
The stone room is in a newly-unearthed annex to Luther's house in Wittenberg.
 Luther is quoted as saying he was "in cloaca", or in the sewer, when he was inspired to argue that salvation is granted because of faith, not deeds.
[...]

The politer terms for merda in Classical Latin were stercus (gen. stercoris), "manure" and fimum or fimus, "filth." Stercus was used frequently in the Vulgate, as in its well-known translation of Psalm 112:7: (Psalm 113:7 in the KJV.)

Suscitāns ā terrā inopem, et dē stercore ērigēns pauperem.

("Raising up the needy from the earth : and lifting up the poor out of the dunghill." DRC)

I question a bit whether the Latin stercus has the same vulgar connotation as the English word shit.  It of course has the same denotation.

It did appear to be slightly on the vulgar side, but perhaps more like "crap" than "shit".  As you can see the difference in degree of vulgarity between "crap" and "shit", so too it's very difficult to know where stercus was on this spectrum of vulgarity in St. Thomas More's day.

But even merda appeared in agricultural and veterinary texts.

Luther Insult Generator:

http://ergofabulous.org/luther/?

I'm pretty sure that's the whole point (which isn't limited to insults).