>>638738
You can't really convince them, unless you read minds and even then it's not 100% sure.
If you have good knowledge and rethoric skills you can argue in favour of God in a good way, but man is not an entirely rational being.
Due to personal circumstances and antichristian ideas in our society many people are emotionally hostile to the very idea of the Christian God, not to mention our Churches. There is no amount of logical reasoning that can fix an heart who hates God, no matter how wrong his idea of God is.
You can't convert them, you can only point them in the right direction, toward God.
The rest is up to God and their own personal conscience.
We have to accept the fact that not all unbelievers are in ignorance, some were called and just said no because "their heart is hardened".
So, you convince them by debating that non material things are possible, what to do then?
Since they accept the possibility then why not suggest they pray, even a small little bit?
The only thing that can change an hardened heart is experience, maybe God will show them a little light and they will understand, or maybe they will close their eyes even more. It's up to them.
tl;dr tell them to pray
God calling them >>>>>>>>>> explaining and arguing