Suppose I thank you with sincerity and expansiveness for saving my life at the risk of your own, and continually praise you to others, trying to get the President to give you a medal. But you didn't actually do anything like saving my life. I am just quite mistaken. Surely you, like any other virtuous person, would be dying of embarrassment and would be doing your best to convince me that you had not done this and hence do not deserve the thanks and praise.
Of course, it is crucial that the praise and thanks be sincere. A virtuous person need not allow himself to be manipulated by insincere fulsomeness. And there will be exceptions. If you thought that my own mental state was too fragile to hear the truth, or if I was too irrational in my belief, you might leave me to my mistake. If you hadn't saved my life but unbeknownst to me had done something else for me that was of the same sort, then you might leave me mistaken as to the exact nature of what you did for me. And, finally, if you haven't yet saved my life, but have an opportunity to do so, you might save my life now or soon instead of correcting me. This would be especially true if you wanted a loving relationship with me, for a love based on such a mistake is little better than a forced love.
The fact that a virtuous person does not contradict great thanks and praise by people who are sincerely convinced that he has made a great sacrifice for them is strong evidence that he has made, or is going to make, either that sacrifice or one of at least the same order of magnitude. And if the praise and thanks comes from people who are rational and psychologically healthy, the evidence is even stronger. And, finally, the simplest explanation of why the virtuous person does not contradict the praise and thanks is that not just that he has made or is about to make a sacrifice of the same order of magnitude, but that he has made the very sacrifice he is being thanked for.
But millions of Christians have praised and thanked God for saving them from sin at the cost of death on the cross, and have not found God to contradict them. And many of these Christians have been quite rational and psychologically healthy. Assuming that God exists—this argument needs to assume that—this gives significant evidence that God did what he is thanked for doing. So, likely, what they thank God for doing is just what God has done.
This argument cannot be used against Christianity since no other religion praises God for a good of a higher order of magnitude. Indeed, it seems unlikely that God could do anything of a higher order of magnitude for us.