Scripture repeatedly warns against the Devil and his angels of evil, aka “demons” or “unclean spirits”

“Stay alert! Watch out for your great enemy, the devil. He prowls around like a roaring lion, looking for someone to devour.” (1Pet 5:8, NLT)

“For if God did not spare angels when they sinned but sent them to Tartarus….then the Lord knows how to rescue the godly from trials, and to hold the unrighteous for punishment on the day of judgment.” 2Pet 2:4a, 9

“Now the Spirit explicitly says that in the later times some will desert the faith and occupy themselves with deceiving spirits and demonic teachings” 1Tim 4:1

It’s clear that if you don’t know your enemy you will succumb to your enemy. To say that the Devil and his demons are not real, external beings also undermines the ministry of Jesus which, as we know, for the most part was spent battling against them.

At the very least implies Jesus was really mad, i.e., talking to himself or having sinful thoughts (something the NT clearly says Jesus never did, Phil 2:5-9; 1Pet 2:22). And at worst, Jesus lied if he was going around pretending to interact with non-existent pagan entities and made up Jewish myths, legends.