For a theist to argue an atheist, they must first prove their God is the one

You can't argue theism vs atheism especially saying that too many people believe in a God, and ask if all of them are wrong. A Christian person first must prove why their religion is the true religion as opposed to Islam, Hinduism etc. Furthermore, they must prove why their specific belief (Protestant, Baptist etc.) is the right interpretation of God. You cannot argue the presense of a Christian God, when Christians themselves cannot settle on set beliefs. Same goes for other religions.