We must stop teaching things that are not true just because it sounds better to our flesh, and that is exactly what the teaching does that says a Christian cannot have a Demon.
Demonic activity is on the rise today simply because we teach dogmas that don't address the problems that demons create in our lives, as demons will surly take advantage of any open door in a believers life!
Dale P. Kruse wrote this very good theological treatise so I offer it to you to clear the air on this issue! It is obvious that much depends on your definition of possession or influence which I am aware of because depending on who you ask ether word can be similar in meaning.
I believe that to be Possessed means to be under control to one degree or another by an evil spirit, the Dictionary meaning is
But influence means this:/ˈɪnfluəns/ Show Spelled noun, verb, -enced, -enc·ing.
God is always the subject and his people are always the objects of the deliverance. The temporal deliverance so dominant in the Old Testament falls into the background in the New. However, the historical accounts in the Old Testament serve as proof that God is the great deliverer.
For example, after recounting examples of God's deliverance, Peter concludes that "the Lord knows how to rescue godly men from trials and to hold the unrighteous for the day of judgment" ( 2 Peter 2:9 ). God still delivers his people from deadly peril ( 2 Col 1:10 ; 2 Tim 4:17 ) and from wicked men ( Acts 12:11 ; 2 Thess 3:2 ).
The dominant idea in the New Testament is God's deliverance from humankind's greatest fears: sin, evil, death, and judgment. These more theological usages closely align with the biblical terms for salvation and redemption.
Believers are to pray for deliverance from the threat of evil that dominates the world ( Matt 6:13 ; Luke 11:4 ). By God's power, believers are delivered from "this present evil age" ( Gal 1:4 ) and the power of Satan's reign ( Col 1:13 ).
What is a devil?
(Greek) "the accuser" or "the slanderer" (Job_1:6-11; Job_2:1-7; Rev_12:10). In Hebrew Satan means "adversary." The two-fold designation marks the two-fold objects of his malice - the Gentiles and the Jews.
He slanders God to man, and man to God (Genesis 3; Zechariah 3). His misrepresentation of God as one arbitrary, selfish, and envious of His creature's happiness, a God to be slavishly-feared lest He should hurt, rather than filially loved, runs through all pagan idolatries.
As "unclean" they can tempt man with unclean thoughts. They and their master Satan are at times allowed by God to afflict with bodily disease (Luk_13:16): "Satan hath bound this woman these eighteen years" with "a spirit of infirmity," so that she was "bowed together."
Our word "panic," from the idol Pan, represented as Satan is, with horns and cloven hoofs, shows the close connection there is between the idolater's slavish terror and Satan his master. The mixture of some elements of primitive truth in paganism accords with Satan's practice of foiling the kingdom of light by transforming himself at times into an "angel of light."
Demons are "his angels" (Mat_25:41; Rev_12:7; Rev_12:9). Natural science can give no light when we come to the boundary line which divides mind from matter. The Bible-asserted existence of evil among angels affords no greater difficulty than its manifest existence among men.
In Mat_4:24, "those possessed with demons" are distinguished from "those lunatic" (probably the epileptic, but even this caused by a demon: Mar_9:14, etc.).
Seeing that physical disease itself is connected with the introduction of evil into the world, the tracing of insanity to physical disorganization only partially explains the phenomena; mental disease often betrays symptoms of a hostile spiritual power at work.
(1) He is deprived of his heavenly excellency, though still having access to heaven as man's accuser (Job 1-2), up to Christ's ascension. All we know of his original state as an archangel of light is that he lost it through pride and restless ambition, and that he had some special connection, possibly as God's vicegerent over this earth and the animal kingdom; thereby we can understand his connection and that of his subordinate fallen angels with this earth throughout Scripture, commencing with his temptation of man to his characteristic sin, ambition to be "as gods knowing good and evil;" only his ambition seems to have been that of power, man's that of knowledge.
(2) From Christ to the millennium he is judicially cast out as "accuser" of the elect; for Christ appearing before God as our Advocate (Heb_9:24), Satan the accusing adversary could no longer appear against us (Rom_8:33-34). He and his angels range through the air and the earth during this period (Eph_2:2; Eph_6:12).
(3) He is bound at the eve of the millennium (Rev_20:1-3). Having failed to defeat God's purpose of making this earth the kingdom of Christ and His transfigured saints, by means of the beast, the harlot, and finally Antichrist, who is destroyed instantly by Christ's manifestation in glory, Satan is bound in the bottomless pit for a thousand years during which he ceases to be the persecutor or else seducer of the church and "the god and prince of the world" that "lieth in the wicked one."
(4) At its close, being loosed for a while, in person Satan shall head the last conspiracy against Christ (permitted in order to show the security of believers who cannot fall as Adam fell by Satan's wiles), and shall be finally cast into the lake of life forever (Rev_20:7-10).