Well, I believe that St. Francis of Assisi came from a non-noble (but well to do) family.
In any case, considering the fact that often times details of their lives are often sketchy (particularly before around 1000 AD) it would be difficult to make such a blanket statement. Even after 1000 AD, it is not unusual for few reliable details to be recorded about a saint’s early life prior to conversion. In fact, there were a number of noble saints, but mainly because those willing to give up a life of privilege for a life of service were bound to attract attention.
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Bill