Let us consider the orthodox belief that a mantra is a rahasya or secret that should not be revealed to others.
The word mantra is derived as: “mun thraayathe ithi manthrah” – a mantra is so called because it protects the mind. A mantra helps us to attain release from the grip of the ego-controlled mind and proceed on our spiritual path without obstacles/distractions. The wording and sound of all mantras are public property. Yet the mantra itself is considered as rahasya or secret. The nearest example:- an idol can be seen or even be possessed by us; but, we can realize the deity represented by the idol only if we associate with it certain sanctity, divine characteristics.
The mind, which is to be controlled and overcome must itself first crave a need for such release, and accept the Mantra as an effective aid to the necessary process. It is to develop such implicit acceptance that one approaches a Guru and receive from him with faith, devotion and love the Mantra as a precious gift, to be treasured and practised. In this sense, the devotee's mantra becomes personal and a rahasya or sacred secret to him/her; it is not a possession to be guarded from others.
Such a mantra per se cannot be reduced in its efficacy or be corrupted in any manner because it is handled differently by others. A truly humble Bhakta who keeps the mantra's divinity in himself and treasures it and practises it will not be concerned with how the mere words of the mantra are used/abused by third parties. Perhaps, Sri Ramanujacharya, the great saint of Sri Vaishnavism, was in great ecstasy when he decided to share the knowledge of his precious possession of the Ashtaakshara Mantra (Om Namo Narayanaya) and declared the Mantra to an assembly of all persons without reference to caste or sex.