He was most likely influenced at least to some degree by the Essene sect. These were Jews who were dissatisfied or disenfranchised by the Sanhedrin in Jerusalem. Mainstream Judaism would have been the theology of the sanhedrin. Johns theology would have been much more austere, and severe. He denounced them for their corrupt practices and lifestyle. You could simply say that his philosophy was a strict adherence to jewish law. Although he participated in the practice of baptism, which was something alien to mainstream judaism. So close to the Essenes, but not exactly. Though essenes were a little too political and no more about the spirit of the law than the Pharisees and Sadducees.