In their mouth no lie was found: this translates the Greek text quite literally, but it is most unnatural English. Something like “they never lied” or Good News Translation “They have never been known to lie” expresses the meaning in a more natural way. This may be the writer’s way of saying that these people never denied they were Christians, as some may have done in an attempt to avoid persecution and martyrdom. The translation, however, must state quite precisely what the Greek text says. On the translation of lie see comments on “false” in 2.2. Some languages will express lie idiomatically; for example, “weave the mouth” (Yapese).

For they are spotless: this has to do with moral or spiritual purity. The same Greek word is used of Christ in Heb 9.14 and 1 Peter 1.19, and of people in Phil 2.15; Col 1.22; Jude 24. Some English translations have “blameless”; New American Bible, Revised has “unblemished,” and Barclay translates “faultless in their purity.” One may also express this as “they have never done any evil things.”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .