The reading "καιπερ εστιν (and yet is)" is considered to have entered the Textus Receptus due to a transcription or printing error. The readings to be found here in extant manuscripts are "και παρεσται (and shall be present)" (A, 025, 046, 051, 1006, 1611, 2053, 2062) or "και παρεστιν (and is present)" (2814, Aleph2, 181, 241, 336, 1854, 2019, 2036, MA). There is no extant manuscript having "καιπερ εστιν" as in the Textus Receptus. However, the Codex Sinaiticus has "και παλιν παρεϲτε," which means "and again present" (being translated “is yet present” by H. T. Anderson in his translation of Sinaiticus), which conveys the same sense as the Textus Receptus reading. Seeing that an ancient text such as Sinaiticus has a variant that agrees with the sense of the Textus Receptus, it would be presumptuous to say that the Textus Receptus publishers did not have any Greek manuscript support for “καιπερ εστιν.” Moreover, the Vulgate omits a clause here altogether, simply reading, "erat et non est" (was and is not)". With there being a fair number of disagreements here even among the more "reliable" witnesses, one cannot dismiss the Textus Receptus reading so quickly.
Please read the article at the following link for a prerequisite to understanding why the Textus Receptus departs from the Nestle-Aland and Byzantine Majority texts in the Book of Revelation: Book of Revelation in the Textus Receptus.
At first the phrase "is not, and yet is" appears to be a contradiction. However, such a paradoxical description is the most fitting for the Beast. The consensus is that the Beast is Babylon in some sort of aspect or form, or by extension (i.e. a king of Babylon) (for the sake of argument, we will not get into the difference between religious and political Babylon, nor try to identify exactly what Babylon is). Babylon can be said to be "the beast that was, and is not, and yet is." Babylon is an ancient city that was conspicuous during its prosperity ("was"), then declined to near obscurity by John's day ("is not"), yet remained influential as a spiritual and/or political system throughout history ("yet is"). Babylon has never truly gone away. It will continue to exist deceptively until the time of the end. 1 Thessalonians 2:6-8 describes the "man of sin" as someone who somehow exists in the present ("yet is") but has not been revealed yet ("is not"). Perhaps that is the meaning of "is not, and yet is".
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