And in their mouth was found no guile; no lie (Revised Version). They had not suffered themselves by self deceit (the second beast) to be beguiled into worship of the first beast—the world. Alford very appropriately refers to Psalms 15:1, Psalms 15:2, "Lord, who shall abide in thy tabernacle?
who shall dwell in thy holy hill? He that walketh uprightly, and worketh righteousness, and speaketh the truth in his heart." For they are without fault before the throne of God; they are without blemish.
The following phrase is omitted by nearly every authority. The word ἀμώμος, "without blemish," reminds us of the "Lamb without blemish" (cf. 1 Peter 1:19; Hebrews 9:14). Thus again they receive appropriate reward.
While on earth they kept themselves undefiled; now they are, like the Lamb, free from blemish (see on Psalms 15:4).