In common parlance Sadhu and Saint are two words used together as well as interchangeably. Sadhu is a short form of Sadhaka. Disciple is a proper English word for Sadhaka. Disciple is someone who practices some sort of discipline under a Guru. Sadhu stands for a saintly person in general. In exact terms –harnessing energies for some great goal is discipline and it includes but isn’t limited to controlling body and mind. Thus Sadhu is a very disciplined person especially someone who practices spiritual discipline.
The discipline is Sadhana. It’s introversion. Extroverted energies are made introverted under some expert guidance in order to do inner work which leads to harmony and peace. The guidance or Guru could be a person, many persons, books or Self which acts via inspiration and intuition.
Siddhi is a term generally used for occult powers and Guru advises to be aware of their hazards. They’re like beacons of light on the dark path towards light giving hints about progress but with a great potency to lure a disciple into booby traps. Siddhi in the truest sense is peace of Self which when gained dissolves all doubts and questions altogether. The Sahaja Samadhi means effortless natural balanced state of mind which is everlasting. The goal of a disciple is Sahaja and it’s normal state of a Siddha. A Siddha is an accomplished master or sage. A sage is also called Saint. Saint, Buddha, Enlightened, Jeevanmukta and Master are various terms used for Siddha in spiritual literature.
Sadhaka is often curious about the state of mind of a Siddha. All masters advise focusing on discipline rather than pursuing vicious and useless curiosities. Jeevanmuktas have no doubts about their mind or about the Truth. Many people believe that a sage is not perfect unless his body disappears into the light as against being left after his departure from this world and many other believe that enlightenment can be judged by some outer signs or behavior. Many people believe that Sadhaka merges into the Self after realization and physical death happens–which is absurd because no knowledge about mind of liberated beings can be had if it were so! Many people think that miracles are telltale hallmarks of a true Saint. Roman Catholics have set criteria of miracles for recognizing personalities as saints. A Saint will never be worried about all these things because he doesn’t regard himself as body-mind. As for miracles–they might or might not happen but a sage is never a showman simply because there are no others to show anything. Ramana said that deliberate miracle performers are worse than magicians showing tricks because magicians know that there is no real magic–only sleight of hand which bemuses onlookers but miracle doer deceives himself by assuming that there are others to witness a miracle and appreciate it.
***Based on a conversation with a friend***