By virtue of the fall into sin, all people are liars (Romans 3:10–18). That is because fallen mankind follows “the deceiver of the whole world” (Revelation 12:9; see further John 8:44). Yet precisely because these 144,000 have been redeemed through Jesus’ blood from their bondage to Satan, their sins (of deceit) have been washed away and their affinity to Satan changed by Jesus’ Spirit so that no falsehood was found in them anymore. As Jesus himself was without deceit (Isaiah 53:9; 1 Peter 2:22), so these redeemed were without deceit (Psalm 32:2; Zephaniah 3:13; John 1:47). Despite being surrounded by the propaganda of the second beast, these redeemed stayed far from whatever was false. So with those same mouths they could (learn to) sing a heavenly song!
5 and in their mouth no lie was found, for they are blameless.