Comments like this in Scripture should not be read in isolation and especially not according to Western Greek philosophical notions of perfection and “without fault.” Romans 3:19-23, in my view, is very important context for Revelation 14:5. It contains probably the clearest explanation of justification by faith in the whole Bible. It has been called the Most Holy Place of the gospel.
The Romans 3 passage is the climax of the first three chapters of the book. Summary these chapters, Romans 3:20 makes it clear that nothing a human being can do earns justification before God. Whatever perfection means, it has to come from a power that is outside of us. All have sinned and continually fall short of the glory of God (3:23). Our inadequacy for salvation is not just a reality of the past, it is a problem in the present and in the future of this life. The present tense of “fall short” (Greek: husterountai) means that all our best efforts, now and continually, still fall short of God’s absolute standards. God’s justification is continuously and freely given to the very ones who “have sinned” and “continually fall short of the glory of God” (Rom. 3:23-24).
In light of Romans 3, “without fault” does not mean absolute perfection, it rather expresses absolute loyalty, the kind where one would rather die than choose to sin. It is not about totaling up our good and bad deeds on a scale. It is about the general trend of our lives. The 144,000 fully and completely trust Jesus to keep their robes clean (7:14) and they are uncompromising in their loyalty to the one who redeemed them (14:4-5).