The premise of this question is wrong. To illustrate that fact simply consider the intent of the law at Deuteronomy. If a man was found out to be a false prophet by means of the criteria provided did that prove that Israel was not God’s nation? Of course not. Israel, in fact, suffered under many false prophets and wicked priests, but ultimately God accomplished his purpose in connection with his chosen people.
As regards Christianity, consider the words of warning provided by the apostle Peter. At II Peter 2:1 we read: “However, there also came to be false prophets among the people, as there will also be false teachers among you. These will quietly bring in destructive sects, and they will even disown the owner who bought them, bringing speedy destruction upon themselves.”
The apostles did not say that there might be false prophets among Christians. He forewarned that there would most certainly be “false teachers among you.” By the same method of reasoning, then, does the existence of false teachers prove that Christ’s congregation does not speak for God?
Furthermore, since there is no evidence that God has ever destroyed any false teachers in the entire Christian era, the destruction that comes upon them speedily must be a future judgment coincident with the coming of Christ. That means that the false teachers will be among Christians up until the day of Christ.