Was "coven" used as a term for a group of witches in 1608 or was another term in use?

Question

I am writing a screenplay set in England in the year 1608. In one sentence I used the word coven (a group of witches), but according to Etymonline this word started to be used from 1660, or 52 years after the events in my story. I need an alternative that conveys the same idea of a congregation of witches doing witchcraft for evil. It must necessarily mean that there are witches involved. Otherwise the meaning of my sentence is lost.

The sentence is as follows: There would be more covens than churches.

Answer 1

No, coven was not used as a term for a group of witches in 1608. According to the OED:

coven noun
Originally Scottish.
1.a. A group of witches which meets regularly; (also sometimes) a meeting of such a group. 1658-
The term first occurs in records of the supposed confessions of those accused of witchcraft in Scotland in the second half of the 17th cent. Following Sir Walter Scott's apparent revival of the word in the work cited in quot. 1830, most 19th- and early 20th-cent. uses were (like Scott's own) in reference to the supposed confessions of Isobel Gowdie in the Auldearn witch trial of 1662 (see quots. 1662¹, 1662²). After c1920 the idea of the organization of witches into covens was, along with the word itself, popularized by the Egyptologist and folklorist Margaret Murray (1863–1963) in her influential writings on witchcraft (see e.g. quot. 1921). Murray's work also popularized the notion—itself drawn from the Auldearn confessions and contradicted by some other early evidence—that a coven typically consisted of thirteen witches (or twelve ordinary members and one ‘officer’).
_{Source: Oxford English Dictionary (login required)}

But a form of convent (α form couent; β form conuent) was:

convent noun
3. † A group of people assembled or gathered together; a number of people meeting together for a common purpose; a meeting, assembly, convention, etc. Obsolete. a1382–1654
β forms
_{[selected attestation]
1609   These eleuen Witches beginning to dance (which is an vsual Ceremony at their Conuents, or meetings, were sometimes also they are vizarded or masq'd) on the sodaine, one of them missed their Chiefe. —B. Jonson, Masque of Queenes sig. A4v
Source: Oxford English Dictionary (login required)}

Here’s an image of Ben Jonson’s Masque of Queenes passage:

In Daemonologie (James I, King of England, 1597) (referenced by StuartF in the OP comments) you can find cõuentiõs (conuentions):

ARGVMENT.
What are the waies possible, wherby the witches may transport themselues to places far distant, And what ar impossible & mere illusiones of Sathan. And the reasons therof.
PHILOMATHES.
Bvt by what way say they or think ye it possible that they can com to these vnlawful cõuentiõs?

Here’s an image of King James’s Daemonologie passage (1603):

Answer 2

The word coven meaning "band" or "group" or "followers" dates from the 14th century. It has connotations of secrecy and collusion and conspiracy.

So it wouldn't be outright anachronism by any means.

You could say "There would be more witch-covens than churches".

P.S.

Gower uses coven in the sense of a group of people living under one roof.

Thei ben togedre broght
Of o covine, of on houshold.

See lines 1890ff below, from a modern English translation of Gower's "Confessio Amantis" by Brian Gastle, Catherine Carter, and Andrew Galloway

line 1890ff

For this False-seeming always has
For its counsel and company
The dark, untrue Hypocrisy,
Whose word and thought are discordant.
For these sins gather together
In one coven, in one household,
As I am about to tell you.
Of False-seeming, I hardly need
Tell you all its old examples;

Chaucer uses the word in the Prologue of the Canterbury Tales when introducing the Reeve, and there it is used in the sense of guile, treachery, cheating, trickery:

Ther nas baillif, ne hierde ne oother hyne
That he ne knew his sleighte and his couyne.

[There was no bailiff, serf or herdsman that he [i.e. the reeve] didn't know their guile and their dirty dealing.]

See the Middle English Dictionary for additional attestations.

P.P.S. Following up a line of thought from the comments, the word coven is quasi-abstract as it refers to "a coming together of people" and so its meaning could range from the rather concrete, as in Gower's "in one household" to Hoccleve's rather abstract "heathens" via an adjective, hethenly couyne:

Thow haast maad a fair permutacion..Fro cristen folk to hethenly couyne.

It could mean something like "posse, homeys, followers" as when Gower uses it to personify pestilence, famine, poverty and woe in the company of personified war:

Dedly werre hath his covine Of pestilence and of famine, Of poverte and of alle wo.

In the Elizabethan period, in his Latin-English dictionary of 1606, Thomas Thomasius glosses a similar abstract-or-concrete word coitio as "assembling, a meeting together : covine or confederacie in doing anything [my emphasis]: a communion : carnall copulation ".

I've emphasized "in doing anything" to show that Thomasius was presenting the idea that the word coitio could apply to any kind of coming together of two or more people for any purpose under the sun. Much the same was true of "coven". By 1475 it is rubbing elbows with political conspiracy to commit treason:

Camillus..wolde not suffre that the Falistes be defrauded of here contre and cite by unjust menes of treason or fals covyn or undew alliaunce.

Answer 3

Another seventeenth-century word sometimes used to describe a gathering of witches is conventicle. Here are five early matches of that term in association with witches that searches of Early English Books Online return.

From Ralph Knevet, Stratiōtikon. Or A discourse of militarie discipline (1628):

For from beneath I heard an hideous [s]ound, / As i[f] some Earthquake dire had cle[f]t the ground / Or Hell her selfe approach to make one, / In their mischieuous consultation. / So Neptune scourged with the Northwind rores: / Such is the clangour, of a thousand ores / Falling at once vpon the surging waue: / The Witches in their conuenticles haue / Such Musicke, as was this for t'was the noyse / Of the infernall pow'rs, that did reioyce, / To see that Hellish-plot contriu'd, and wrought, / That might bring all the world againe to nought

From a 1629 translation of Ch. de Fonseca, "The XXIII Sermon, Upon the Fryday After the Third Sonday in Lent," in Deuout contemplations expressed in two and fortie sermons vpon all ye quadragesimall Gospells:

As in the place where sinners meet, as in your Conuenticles of Heretickes, and Witches, the Deuill comes amongst them, offering them imaginarie fountaines of delights: So, in holy places thou shalt presently meet with God, who will offer thee fountaines of liuing waters, &c. Tertullian treating of the Amphi∣theaters where men went in to kill one another, sayd, Tot daemones, quot, That there were as many Deuils there, as there were Men. And a woman that was a Christian comming from these sports, the Deuill entred into her. And beeing asked how he durst doe so to a seruant of our Sauior Christ, made answer, I found her within the limits of my Iurisdiction.

From Thomas Heywood, The hierarchie of the blessed angells Their names, orders and offices the fall of Lucifer with his angells (1635):

In briefe, all those operations, Conjurations, Incantations, Abjurations, Murmurations; all those Conuenticles and nightly assemblies in places desart and remote, of Witches, Sorcerers, Magitions, Conjurers, and such like, haue the great Diuell himselfe for their Authour and Abettor.

From H. F., A true and exact relation of the severall informations, examinations, and confessions of the late witches, arraigned and executed in the county of Essex (1645):

Take one in∣stance of the jugglings and illusions ef the Devill above all the rest, which doth most palpably detect him herein, is a History related by Johannes Baptista Porta, in his second book, de Magia naturali; hee there witnesseth, that upon the Devils suggestion, a Witch believed firmly and perswaded her self, that all the night shee had rid in the aire, over divers great mountaines, and met in conventicles with other Witches, when the same night, the mentioned Author himself, with others that watched her and saw her all that imagined time of her transvection in the aire, to be within her chamber profoundly sleeping; ...

From Henry More, An antidote against atheisme, or, An appeal to the natural faculties of the minde of man, whether there be not a God (1652):

CHAP. VII. The nocturnall Conventicles of Witches; that they have often dissolved & disappeared at the naming of the Name of God or Jesus Christ; and that the party thus speaking has found himself alone in the fields many miles from home. The Dancing of Men, Women and cloven-footed Satyres at mid-day; John Michaell piping from the bough of an Oake, &c.

...

I will only adde a Story or two out of Remigius concerning these Conventicles of Witches, and then I will proceed to some other proofs.

John of Hembach was carried by his Mother being a Witch to one of these Conventicles, and because he had learnt to play on the Pipe, was commanded by her to exercise his faculty & to get up into a Tree, that they might the better hear his Musick.

The OED notes that conventicle was often used in a neutral sense ("a meeting secular or religious") from the fourteenth century forward. However, it also notes early use of the word in less benign senses:

3. A meeting or assembly of a clandestine, irregular, or illegal character, or considered to have sinister purpose or tendency. [Citations from as early as 1383 omitted.]

4. A religious meeting or assembly of a private, clandestine, or illegal kind ; a meeting for exercise of religion otherwise than as sanctioned by the law [Relevant citation from 1526:] He sente a flode after her, by the which is vnderstanded the conuentycle of heretykes.

That this sense of the term was in contemporaneous usage in 1608 is evident from this instance from Thomas Hutton, Reasons for refusal of subscription to the booke of common praier vnder the hands of certaine ministers of Devon, and Cornwall word for word as they were exhibited by them to the Right Reverend Father in God William Coton Doctor of Divinitie L. Bishop of Exceter (1605):

For conclusion of this questiō. First private signifieth, that which is done privily in a clanculary manner by stealth, without authority contrary to Gods word by Jesuits, seminary Priests, schismatical teachers, and the like, in which sort, if any of our brethren meane, they proue nothing against vs, for our church generally condēneth such conventicles, whither of hereticks, or schismaticks.

Although people today may suppose that the primary objection to witches four centuries ago related to their ability to perform uncanny acts of wickedness that harmed others, contemporaneous hostility gave considerable place to witches' religious deviance: they covenanted with and worshiped the Devil rather than God, and in that sense were heretical as well as being dangerously acquainted with curses and other magical powers. Viewed in this light, it seems no great conceptual leap to go from conventicles of heretics in 1605 to conventicles of witches in 1628.