William Burkitt on Revelation 14:5

Here St. John closes the description of the fore-mentioned followers of the Lamb,

1. With the character of their integrity; like the Lamb, no guile is found in their mouth, as there was none in his, 1Pe 2:22 they were free from that hypocrisy which was found amongst anti-christ's followers, who profess to worship God, but adore their idols; sincerity and uprightness of heart towards God and man was found with them.

2. They are said to be without fault before God; which may be understood comparatively; they have no such faults as antichrist's followers are guilty of; they worship God aright, and are approved by him: or, if absolutely, we must understand it of their glorification in heaven, where all the saints are without spot, and blameless before the throne of God, perfectly like unto God and the Lamb, as well in purity as in immortality.