the power and authority of Christ means that evil can not touch that which belongs to Christ.
Not permanent/indefinitely (i.e., the life of the spirit), no, dear one (peace!). However, the life of the flesh?... yes, evil can touch it. Satan "touched" Job's flesh. And those who were "evil" not only stoned Stephen and killed the Prophets, but had my Lord... and many of his disciples... killed in the flesh.
"Demons" (bad/evil spirits) tend to try and torment the spirit. They do this in order to take us to the point that we CHOOSE death (of the spirit) rather than [everlasting] life. By the authority of Christ, however, we can, through holy spirit, oppose... and even "conquer"... such tormentors, these "bad" spirits/demons. We can do it even with the Adversary.
Again, peace to you!
YOUR servant and a slave of Christ,
SA