Some physicists seem to think that the only good reply to the fine-tuning argument for God is an appeal to many universes. If that’s right, that puts the fine-tuning argument on pretty strong ground. Leonard Susskind is a physicist who falls into this camp. He says:

If, for some unforseen reason, the landscape [i.e., the many-universes version of string theory] turns out to be inconsistent — maybe for mathematical reasons, or because it disagrees with observation — I am pretty sure that physicists will go on searching for natural explanations of the world. But I have to say that if that happens, as things stand now we will be in a very awkward position. Without any explanation of nature’s fine-tunings we will be hard pressed to answer the ID critics. 

In fact, I think that the many-universes reply to the fine-tuning argument isn’t as strong as many people think, but there are other stronger ways of responding to the fine-tuning argument that don’t depend on there being many universes. For my defense of this, see my paper “God, Fine-Tuning, and the Problem of Old Evidence”. I wonder if Susskind has actually gone through the various other responses one could give to the fine-tuning argument, and concluded that they all are bad, and hence the many-universes reply is the only good one, or if Susskind has simply latched on to the many-universes reply, and hasn’t really thought about other responses.