Although I have been using atheists as providers of meatsuits I do think that there are others who could be suitable. An example would be the hypocritical lying politicians that all nations are cursed with, the sort that will happily be photographed in front of a church during an election campaign and once in government will happily support abortion and economic creeds that push people into poverty for political gain and fun.

In that case you can simply push any demon in because there would be no contract of sorts between you and that demon, whereas if you are looking to inject a demon into an atheist so as to provide that demon a chance to seek redemption there would be some sort of agreement before the possession took place.

If there is some sort of reciprocal agreement in relation to assisting with a possession there would be a degree of understanding and indeed mutual respect. The relationship between the human and the demon sometimes was known as the witch (human) and familiar (demon), but generally such a relationship required the consent of that demons superior (sub prince) who would order a servant demon to serve the witch and assist the witch in magical matters. In this setup the demon remained a spirit and was not a physical being, but the downfall in such relationships occurred when they began sleeping with each other which just shows that the familiars did have the ability to possess humans usually with the assistance of the witch. I suppose that can be compared with a boss who fucks his secretary, sometimes it works but the relationship between the 2 no longer resembles what the original agreement was and that impacts on the business.

So looking at the atheist and the hypocritical politician you have a choice of one without a soul and one who has an utterly corrupt soul. For the utterly corrupt soul there would be a personality change of little note, an arrogant politician is an arrogant politician so there is no standard of decency expected. You could try to get a 'good' demon to bonejack them in which case they probably wouldnt be as corrupt or dishonest as they are now. Or you can get a real bad bastard to bonejack them in which case when the politician is being photographed outside the church and a journalist asks them 'do you find it contradictory that you attend church and support abortion?' And the response is 'fuck you i do what i want and i only come here to get votes and get laid, now suck my dick!'

Either way you win, people will either gain hope or gain a slap in the face to wake up. But lets say you get someone possessed and the demon wants to silence you in order to protect their identity, this is why you must do a new sigil for them on an Earth kamea. You tell them that if they cause you to be killed that the sigil and the operation that allowed them to possess someone will be passed on to the catholic church exorcists and that they will soon be toast. You should arrange such a letter to be sent when you die.

I think it was Nietzche who said 'when you look into the abyss the abyss looks at you' as well as 'when fighting monsters one must ensure one does not become a monster themself'
Myself, I prefer this lyric from a slayer song... 'raise the stake in malice, you cannot plunge it down'
If you want to travel this path you will change.....