If I had more time, I'd offer to train your ass by acting as the atheist myself. I'd ravage your ass with top-grade edginess and massively advanced terrorising weapons of logic and euphoria. You'd have to eat through 50 fedoras to get to my little black heart. It'd be good training.
I must get my own ass to bed before long, or my health is going to be shit for another day. However, I'll write the questions I used to get a lot back on 4chan.
>Dinosaurs?
>If God is good, why will I go to Hell for not obeying Him?
>What about people born in India?
>What denomination are you from?
>Why do you fuck little boys?
>What is the size of your dick?
>Why do you think we care?
And plenty of other questions that aren't always very interesting, but that's where they stop, the atheists, so you must answer them. For dick size and such, either ignore or tell the truth, or say 8 inches or 20 cm. Christians with big cocks will always get you the attention of a few faggots whose fantasy it is to seduce you, because if their sexiness can derail you from your faith, then they sure must be sexy. Don't waste too much time explaining that you're not a pedo. Answer once, then ignore.
As to questions pertaining to "Why are you here?", explain that you are here for anyone who has questions or remarks or wishes to discuss anything related to Christianity or God. Stating this will force them to either stay and have a question or remark or want to discuss, or depart. They can't keep whining about your presence coherently if they are free to leave at any moment. This is why preaching online works better than door to door in today's world. People come to you, not you to them.
Be yourself, you're a channer, you know the lingo, well-meme'd!, so don't try to hide that. Other anons must realise you're one of them. But don't start getting silly, spouting memes for the sake of it. Be natural, say what you'd actually say.
Another thing that will happen 70% of the time is that you will get fucking flooded. By God, you will. When I did these threads, I'd typically answer with a 40-minute delay because I took a long time to answer everyone (you know the average length of my posts). These threads could last 4 hours or more, and for those hours, I never stopped typing. Typing very fast is a good skill to have.
You can reduce the hassle by opening a thread together with other anons. Get together, then go to 4chan, or even /b/ here on 8chan, actually, and get ready to talk to the heathens!
Regardless of where you do it, do not say stuff like "COMING TO YOU STRAIGHT FROM 8CHAN, WE'RE HERE TO SAVE YOUR SOUL!" because that shit is obnoxious as hell. The "official /christian/" thread on /b/ here was a nuisance for this reason alone: you're not there to advertise a board, you're there to share Christ. Boards don't matter by comparison. Don't let people think you care more about your board than your Lord. 50 points for that sick rhyme, nigga.
One thing you can do, at the end of your time, is point the curious to some boards if they want further advice. Don't do this at the beginning, and only do this once the thread is mostly done or if you're about to leave. Point them to /christian/ or here.
Personally, and you know that's mostly me and nobody else, channers fap a lot, and whatever you believe about fapping, I don't want you to tell them not to fap anymore in the beginning. Just don't, because for many of them, this is about the only good thing they have in life and the idea of losing even this may be an obstacle too big to overcome. I don't want to lose anybody for Christ because of dick obsession. That said, you'll also find many "no fap this or that month", and those might actually be interested in this aspect of the faith. But yeah, do not make this a dogma upon them, at least not in the beginning. Despite what you might think, I virtually never mentioned masturbation back in my preaching days on the old chan. I still believe sexual matters should not be made central when preaching to the Godless. Doing so will focus the whole thing on sex and God will soon be forgotten, so don't. Leave the sex stuff for later if you must. In the meantime, use what's good about it: like that priest who said that the problem with porn is that "it shows too little!", meaning that you don't get to see loving intimacy in porn (by definition you can't) and such things. If you start talking like a psychorigid Mother Superior, nobody will listen.