How can you recognise a true King James Onlyist? There are a number of ways, but one of the best is the Easter test. In Acts 12:4 the New King James reads, "So when he had arrested him, he put him in prison, and delivered him to four squads of soldiers to keep him, intending to bring him before the people after Passover." The King James says, "And when he had apprehended him, he put him in prison, and delivered him to four quaternions of soldiers to keep him; intending after Easter to bring him forth to the people." Aside from the King James Bible's use of the technical term 'quarternion', the thing that stands out is that in the KJV we have the word 'Easter', while the NKJV has 'Passover'. Now, the two translations come from the same Greek text, and besides, there is no manuscript variant here; the Greek is the same in every manuscript of Acts 12:4.
that is why this is a test of King James Onlyism; the variation is in the English, not the Greek. A person who makes this a litmus test of orthodoxy has made the English text the test, and is claiming, at least by implication, that the KJV is actually superior to the Greek.
The Greek word here is 'Pascha', the same word that is translated 'Passover' everywhere else in the New Testament. However, the word 'Passover' was not coined in English until William Tyndale translated the book of Exodus, some years after he made his English translation of the book of Acts. While the Wyclifite translation followed the Latin in transliterating the Greek as 'Pasch', Tyndale decided to use the word 'Easter', referring of course to the Christian festival of the resurrection of Christ, and does so throughout his New Testament. In Tyndale's New Testament Acts 12:4 reads, "And when he had caught him he put him in preson and delyvered him to .iiii. quaternios of soudiers to be kepte entendynge after ester to brynge him forth to the people." However, when he turned to the Old Testament he realised that it would be a massive anachronism to use 'Easter' before the birth of Christ, and so he coined, probably following the German, the word 'Passover'. He was martyred before he could revise the New Testament. Thus Coverdale's 1535 Bible reads, "Now whan he had taken him, he put him in preson, and delyuered him vnto foure quaternions of soudyers, to kepe him: and thought after Easter to bringe him forth to the people." The Bishops' Bible is practically identical, "And when he had caught hym, he put hym in pryson also, and delyuered hym to foure quaternions of souldiers to be kept, intendyng after Easter to bryng hym foorth to the people." Spelling varies because English spelling was still largely phonetic and not for any other reason.
The Geneva Bible, in 1587, was the first English Bible to change 'Easter' to 'Passover' in the New Testament, and did so consistently, so that in the Geneva Acts 12:4 reads, "And when he had caught him, he put him in prison, and deliuered him to foure quaternions of souldiers to be kept, intending after the Passeouer to bring him foorth to the people."
It must be remembered that the King James Bible was intended by the King himself to be a revision of the Bishops' Bible rather than an entirely new translation, so that it is most correct to say that 'Easter' in Acts 12:4 was retained, not introduced into the text. Why we do not know; it might even have been just an oversight on the part of the translators and not deliberate at all. The point is that until 1587 every English Bible translated from the Greek used 'Easter' to translate Pasch.
There are very few King James Onlyists who will explicitly say that they think that the English of the KJV is the standard and trumps the Greek; this verse, because it is not a textual variant but a translational oddity, exposes the truth. And the KJV-Onlyists can camp out here, denouncing anyone who thinks that the word should be rendered the way it is everywhere else in the KJV. There are various crank theories that will be trotted out, but the answer to them all is that there is nothing in the text that suggests an alternative translation. To insist that Acts 12:4 should say 'Easter' is to set the KJV above the Textus Receptus, and indeed every Greek text.