The devil; Satan
In our New Testament (NT), "the devil" is first mentioned in Matthew 4 and Luke 4; Mark, in his comparable, very brief mention, has the name “Satan” in place of “devil” (1:13). The setting is Jesus’ “temptation”: Will Jesus pay heed to the evil one's prompts to take the easy way? (I suppose under the conventions of writing it makes sense to capitalise the word that seems to be used as a name.)
In the pages of the NT, evil forces are taken seriously, but not feared, by those who follow Jesus.
There came a stage when Jesus expanded his reach in his community by delegating and sending out. This involved followers confronting the works of evil: Summoning his twelve disciples, he gave them authority over unclean spirits, to drive them out and to heal every[a] disease and sickness.[b]...
Heal the sick, raise the dead, cleanse those with leprosy,[a] drive out demons. Freely you received, freely give. ...
If anyone does not welcome you or listen to your words, shake the dust off your feet when you leave that house or town. Truly I tell you, it will be more tolerable on the day of judgment for the land of Sodom and Gomorrah than for that town. (Matthew 10: 1,8, 14-15, CSB). Mark and Luke have comparable reports. I note that "spirit" and "demon" are used as equivalents (see further following).