What does it mean that God's children will not lie?
Answered in 1 source
It means they will speak truthfully about their need for grace and reject false doctrines that undermine the Gospel.
The phrase 'children that will not lie' emphasizes that true believers in Christ will acknowledge the reality of their sin and the truth of God's word without distortion. They will not be swayed by doctrines contrary to the Gospel, such as the idea that God’s love is universal or that salvation is a choice stemming from human will. This idea highlights the believer's recognition of the fullness of grace and the reliance on the finished work of Christ. As stated in Scripture, believers affirm that Christ is their only hope and that they are saved by His grace alone, consistently rejecting any semblance of self-righteousness or boasting in their works (1 Timothy 4:1).
Commentary