"Nothing but" has the same meaning as "only", used for emphasis of the object.
E.g. "The rich man eats nothing but the best food available in the restaurant" = "The rich man eats only the best food available in the restaurant".
"Nothing but a miracle can prevent the accident from happening" = "Only a miracle can prevent the accident from happening".
"He is nothing but an old man, so he could not defend himself against the gang of robbers" = "He is only an old man, so he could not defend himself against the gang of robbers".
