Yes, all humans who have not been baptized or are in the state of mortal sin are under the dominion of the Devil. That is a dogmatic fact. The more hideous the sins, the more demons can dwell in the soul; but at the very least, for people who are in not in the state of sanctifying grace, theirs souls are dead.

~Cantarella

From Teresa of Avila;

1. BEFORE going farther, I wish you to consider the state to which mortal sin46 brings this magnificent and beautiful castle, this pearl of the East, this tree of life, planted beside the living 46 waters of life47 which symbolize God Himself. No night can be so dark, no gloom nor blackness can compare to its obscurity. Suffice it to say that the sun in the centre of the soul, which gave it
such splendour and beauty, is totally eclipsed, though the spirit is as fitted to enjoy God’s presence as is the crystal to reflect the sun.48 2. While the soul is in mortal sin nothing can profit it; none of its good works merit an eternal
reward, since they do not proceed from God as their first principle, and by Him alone is our virtue real virtue. The soul separated from Him is no longer pleasing in His eyes, because by committing a mortal sin, instead of seeking to please God, it prefers to gratify the devil, the prince of darkness,
and so comes to share his blackness. I knew a person to whom our Lord revealed the result of a mortal sin49 and who said she thought no one who realized its effects could ever commit it, but would suffer unimaginable torments to avoid it. This vision made her very desirous for all to grasp
this truth, therefore I beg you, my daughters, to pray fervently to God for sinners, who live in blindness and do deeds of darkness. 46

3. In a state of grace the soul is like a well of limpid water, from which flow only streams of clearest crystal. Its works are pleasing both to God and man, rising from the River of Life, beside which it is rooted like a tree. Otherwise it would produce neither leaves nor fruit, for the waters of 47 grace nourish it, keep it from withering from drought, and cause it to bring forth good fruit. But the soul by sinning withdraws from this stream of life, and growing beside a black and fetid pool, can produce nothing but disgusting and unwholesome fruit. Notice that it is not the fountain and the brilliant sun which lose their splendour and beauty, for they are placed in the very centre of the soul and cannot be deprived of their lustre. The soul is like
a crystal in the sunshine over which a thick black cloth has been thrown, so that however brightly the sun may shine the crystal can never reflect it.
4. O souls, redeemed by the Blood of Jesus Christ, take these things to heart; have mercy on yourselves! If you realize your pitiable condition, how can you refrain from trying to remove the darkness from the crystal of your souls? Remember, if death should take you now, you would never
again enjoy the light of this Sun. O Jesus! how sad a sight must be a soul deprived of light! What a terrible state the chambers of this castle are in! How disorderly must be the senses—the inhabitants of the castle—the powers of the soul its magistrates, governors, and stewards—blind and uncontrolled as they are! In short, as the soil in which the tree is now planted is in the devil’s domain, how can its fruit be anything but evil? A man of great spiritual insight once told me he was not so much surprised at such a soul’s wicked deeds as astonished that it did not commit even worse sins. May
God in His mercy keep us from such great evil, for nothing in this life merits the name of evil in 48 comparison with this, which delivers us over to evil which is eternal. 5. This is what we must dread and pray God to deliver us from, for we are weakness itself, and unless He guards the city, in vain shall we labour to defend it.50 The person of whom I spoke51 said that she had learnt two things from the vision granted her. The first was, a great fear of offending God; seeing how terrible were the consequences, she constantly begged Him to preserve her from falling into sin. Secondly, it was a mirror to teach her humility, for she saw that nothing good in us springs from ourselves but comes from the waters of grace near which the soul remains like a tree planted beside a river, and from that Sun which gives life to our works. She realized this so vividly that on seeing any good deed performed by herself or by other people she at once turned to God as to its fountain head—without whose help she knew well we can do nothing—and broke
out into songs of praise to Him. Generally she forgot all about herself and only thought of God when she did any meritorious action.

http://www.docuмentacatholicaomnia.eu/03d/1515-1582,_Teresa_d%27Avila,_The_Interior_Castle_Of_The_Mansions,_EN.pdf