Imagine

With the lifelong believer, you can try the argument, "Imagine there's no God," but it won't work.
Religious faith has become such a part of people's identities that they are unable to imagine there's no God.
They simply cannot.

They will tell you they can imagine a godless universe but they can't. They might imagine in their minds the emptiness of space, or a windowless room with no lights, or perhaps a person devoid of happiness and hope, as that is what they might consider life to be as a godless person.
But they can't imagine what it is like to not believe there is someone looking after you, that you are not alone, that someone will always be there to help.

I don't believe the strongest believers know how to stop being faithful even when they try. All their lives they have imagined the presence of God and their brains are actually shaped by this belief. Neural pathways formed early have been strengthened by a lifetime of use.
Those who are programmed to believe, either by themselves or through childhood indoctrination, are usually believers for life.
It takes a very strong personality to break free of a lifelong fervent belief.
Most never will.

I think it's easier for a non-believer to imagine a God than for a believer to imagine there isn't. (I'm talking about a non-believer who has never believed.)
As there's no proof for God, all it takes to imagine there is a God is pretending that God is apart from the natural universe, that he created the universe, and that he controls the universe.
Then imagine yourself making decisions based on what God wants, rather than what you want.
I was once married, so it's easy enough to imagine trying to please someone else that gets angry if you don't.
I was once a child, so it's easy to imagine a parent as a total authority figure on whom you rely utterly for life and happiness.

But living your life for God is much harder to ignore when you try to imagine there is no God. You'd be accustomed to making decisions for God, to asking for direction from God, to praying for yourself and your loved ones.

When you ask a lifelong believer to, "Imagine there's no God," you are likely to get a faith based response.
"I can't imagine that!"
"I have too much faith for that."

Or you might get some misdirection response that tells you that person is incapable of even considering the notion.
"Imagine God is in your heart."
"Why would I want to?"

Give it a try next time you're debating a believer. I bet you'll get some kind of evasive response.

And don't get mad if they won't try to imagine there's no God... they don't know how.

They can't.