The clause "and art to come (και ο ερχομενος)" is omitted in most modern translations that follow the Nestle-Aland or Byzantine Majority texts. Please read the article at the following link for a prerequisite to understanding why the Textus Receptus departs from these modern texts in the Book of Revelation: Book of Revelation in the Textus Receptus.
The complete omission of the clause is supported by Alexandrinus (5th century), a seventh century revision to Sinaiticus, and most Byzantine manuscripts. While the original hand of Sinaiticus (4th century) and P47 (3rd century), the two earliest manuscripts of Revelation, do not have "ο ερχομενος", they agree with the Textus Receptus in having the "και (and)" after "ο ων και ο ην (which art, and wast)". Ephraemi Rescriptus (5th century), 2344 et al., and Vulgate manuscripts also have the extra "και". Agreeing completely with the Textus Receptus are 1841 (9th/10th century), 051 (10th century), 1006 (11th century), et al. and Tyconius (4th century). Thus the only manuscripts that are earlier than the earliest manuscript with the Textus Receptus reading (i.e. minuscule 1841) are P47, Sinaiticus, Alexandrinus and Ephraemi Rescriptus. Three of these do not even remove the clause completely because they retain the "και" after "ο ων και ο ην". Thus there is only one manuscript from before 1841 that has the exact wording of the Nestle-Aland text.
The "και" after "ο ων και ο ην" in P47, Sinaiticus, and Ephraemi Rescriptus makes for awkward syntax. Its inclusion suggests that the entire clause "ο ων και ο ην και ο ερχομενος" was original and a scribe omitted only "ο ερχομενος" and left the "και". If the original did not have the "και", there is nothing in the context that would have compelled a scribe to add it. On the other hand, a scribe would have had a contextually motivated reason to omit the present participle. The context seems to suggest that the Lord had already come (Revelation 11:15). However, inclusion of the present participle does not offend the context because verse 15 only fulfills a claim to the temporal realm and Christ actually comes later at Revelation 19:11-16. The 1904 Patriarchal Text of the Greek Orthodox Church has chosen to side with the Textus Receptus reading and the Clementine Vulgate also has the clause ("et qui venturus es").
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