Most of us were athiests and they have already rejected the other religions, they will be the people that are the easiest to show the truth out of all non Spiritual Satanists.
If one of them isn't ready to be directly confronted with the truth, like in the case of atheists who deny the existence or even the possibility of anything spiritual the best thing to do is to very sublimaly send them in this direction.
For atheists, no matter how zelous they reject all spirituality any stimulation of their aura will make them wonder if there is more to it than what they believe. Humans are all curious, so try to get them to think about it themselves. If you are close to a person like that, like a good friend or close famely member, perhaps your parter you can spark up a conversation about meditation or something. You don't need to say you are a spiritual satanist, but just tell them you have been trying to meditate lately. They will be surprised if they don't already know this, you can tell them it felt surprisingly good, it helped you consentrate and stuff.
This will spark their curiousity in most cases and with enough thought they will find their way to some spiritual websites, they might ask you about it themselves.
Just make sure to not scare them off by smacking the truth right in their face. For some people this will work best, but in most cases they are too blinded to understand what you are telling them and they will think you are some crazy floating new age person who's lost it's mind and it's footing..
This is how I first came to Spiritual satanism. My whoole life I had believed in some kind of spiritual things, but I was just as much of an athiest as any other person in the beginning. I kept hearing people talk about this and decided to try meditating, I came across others who were doing the same, I had some nasty experiences with Spirituality and closed my self off for a while, but then I gradually started to come across JoS links and I started to wonder if demons are really the evil beings we are all told they are.
I started to read more about this and from there I came across the JoS pages. At first it was hard to take in, however I kept comming back to it and well now here I am.
I think for most people it will be a reletively slow process like mine, but it depends all on the individual like Call_Me said. The more open minded a person is to start with to easier you can give the truth to them. However, eventually every person that isn't origionally form the enemy will start to open up their minds no matter how stubborn they are. The momen they ask themselves the question: Hmm maybe there is more to it than this? Is the moment they are going to see the truth at one point in their life or another. Once this door is opened there is no going back.
So when a person you know is already asking himself this you can really spark his interest and guide them to JoS.