Name: The Saint

There never was an ACTUAL explanation to it per say. But the reasons always given were his initials and the fact that he always helps those who need it. However, the novels were different as he was a more ruthless character in them, so the name, in the novels, would more then likely come from his initials and his contrary nature as a man out for his good and was likely to commit murder if the situation demanded. The series was a more watered down version of Templar, but the earlier black and white episodes were a lot closer to the Saint of the books.





Be as wise as a serpent and as harmless as a dove

In the Trivia section here it says:

During the writing of his manuscripts, Leslie Charteris constantly designated Simon Templar by his initials (S.T.) in order to save time. That's how the idea came up to give him the nickname "the Saint".

Which makes sense to me.