Like many terms related to spirituality, this one is grossly misunderstood. Although many people claim to be "masters," the truth is that the terrestrial person is only a terrestrial person. The only one who can be a master is the Innermost, Atman, the Father, Chesed.
"And, behold, one came and said unto [Jesus], Good master, what good thing shall I do, that I may have eternal life? And he said unto him, Why callest thou me good? there is none good but one, that is, God." —Matthew 19
“It beseemeth not a man, that God should give him the Scriptures and the Wisdom, and the gift of prophecy, and that then he should say to his followers, ‘Be ye worshippers of me as well as of God;’ but rather, ‘Be ye perfect in things pertaining to God, since ye know the Scriptures, and have studied deep.’ God doth not command you to take the angels or the prophets as lords.” —Koran 3:9
"The value of the human person which is the intellectual animal called human being is less than the ash of a cigarette. However, the fools feel themselves to be giants. Unfortunately, within all the pseudo-esoteric currents a great amount of mythomaniac people exist, individuals who feel themselves to be masters, people who enjoy when others call them masters, individuals who believe themselves to be Gods, individuals who presume to be saints. The only one who is truly great is the Spirit, the Innermost. We, the intellectual animals, are leaves that the wind tosses about... No student of occultism is a master. True masters are only those who have reached the Fifth Initiation of Major Mysteries [Tiphereth, the causal body]. Before the Fifth Initiation nobody is a master." —Samael Aun Weor, The Perfect Matrimony
"You [if you have reached levels of initiation] are not the master, you are only the sinning shadow of He who has never sinned. Remember that only your internal Lamb is the master. Remember that even though your internal God is a Hierarch of fire, you, poor slug, are only a human being and as a human being you will always be judged. Your internal Lamb could be a planetary God, but you, poor slug of the mud, do not forget, always remember that you are only the shadow of your God. Poor sinning shadow..! Do not say “I am this God” or “I am that master,” because you are only a shadow that must resolve to die and be slaughtered in order not to serve as an obstacle for your internal God. It is necessary for you to reach supreme humbleness." —Samael Aun Weor, The Aquarian Message
"Do not accept external masters in the physical plane. Learn how to travel in the astral body, and when you are skillful in the astral, choose an authentic master of Major Mysteries of the White Brotherhood and consecrate unto him the most absolute devotion and the most profound respect." —Samael Aun Weor, The Zodiacal Course