How do we know God's judgment upon the fallen angels is just?

The justice of God's judgment against fallen angels is affirmed through the understanding that He is perfectly holy and righteous. As highlighted in 2 Peter 2:4 and Jude 1:6, the judgment was not arbitrary; rather, it was a direct consequence of their rebellion. Angels, being rational and moral beings capable of knowing God's truth, chose to forsake their positions and align themselves against Him. Therefore, their punishment, which includes being reserved in everlasting chains under darkness, is a reflection of God's unwavering commitment to justice and order.

Moreover, God’s character – being just and holy – necessitates that He cannot let sin go unpunished. His judgment upon the angels serves not only as a punishment for their transgression but also as a warning to humanity regarding the seriousness of sin and the consequences of turning away from God's authority. This understanding bolsters the faith of believers, reinforcing that God's actions are always rooted in His holy nature and righteous decrees, ensuring that no one is above the law of divine justice.