What is a Mantra?
The definition of Mantra is
Mananat Trayate Iti Mantrah
The repetition of which protects or liberates one is called a Mantra (one of many possible translations)
Traditionally Mantra's are the ones commonly chanted by priests. But most saints say that the most powerful mantra is just chanting the name of your Ishta Deva.
Sri Swami Sivananda says in The Secret of Bhakti Yoga by Sri Swami Sivananda:
The name is a verbal expression of the conception of the Supreme Being, who is known by different names and forms, in different climes and times.
Without the name of the beloved none may hope to cross the sea of samsara (worldly existence). The Lord's name is the lasting medicine against any kind of ailment. You cannot be overtaken by sorrows, sins, troubles, sicknesses if you take refuge in the Lord's name. Repetition of the Lord's name dispels all fears and evil thoughts.
What are the (general) benefits of chanting Mantras?
Chanting mantra has three fold benefit
1. Physical benefits:
By constant chanting and repetition of a mantra, your body gets purified and unwanted toxins are removed.
In Power of Chanting by Melanie Cooper, she paraphrases Jonathan Goldman's words in His article, the Science behind Mantra Chanting
He lists physical benefits (of mantra chanting) firstly as relaxation, it lowers heart beat, calms brain waves and relaxes the breath. Chanting balances the functioning of the two sides of the brain and oxygenates the brain.
Because by constant repetition of the mantra, one is knowingly (or unknowingly) regulating one's breathing patterns and heart-rate. Prana, which is another name for the life-force, is very important for proper functioning of the body-mind complex. By regulating your breath and your prana, you are automatically getting physical benefits.
Sri Swami Sivananda says in The Science of Pranayama
If one controls the breath or Prana, the mind also is controlled. He who has controlled his mind has also controlled his breath.
He who practises Pranayama will have good appetite, cheerfulness, handsome figure, good strength, courage, enthusiasm, a high standard of health, vigour and vitality and good concentration of mind.
Note that Pranayama here refers to the specific Yogic technique where the breath is consciously and specifically controlled and regulated. This must not be experimented without guidance by Guru as it could have bad side effects if not done accurately. However, by chanting mantras constantly and sincerely, this regulating of breath and prana occurs naturally and thus does not have any negative side effects.
2. Mental benefits:
By constant chanting and repetition of a mantra, you begin to get pure thoughts, good positive energy flows into your Mind. With great positive thoughts, pure thoughts, divine thoughts, your Mind gets purified.
When the Mind is purified, all the six enemies of the mind - lust, anger, greed, pride, delusion and jealousy - get weakened and lose their grip over you.
Because each Mantra has a meaning and is associated with God. Thus by chanting the Mantra one is knowingly (or unknowingly) thinking benign and positive thoughts, divine thoughts, thereby creating a strong positive force.
Sri Swami Sivananda expounds about the Gayatri mantra repetition in Gayatri Japa:
The mind is purified by constant worship. It is filled with good and pure thoughts. Repetition of worship strengthens the good Samskaras. "As a man thinks, that he becomes. " This is the psychological law. The mind of a man who trains himself in thinking good, holy thoughts, develops a tendency to think of good thoughts. His character is moulded and transformed by continued good thoughts.
3. Spiritual benefits:
By constant repetition of a Mantra, one develops renunciation, which leads to liberation, which leads to self-realization or to God.
Paramhansa Yogananda says in Cosmic Chants:
Holding the attention unbrokenly on the real Cosmic Vibration, Aum or Om, not on an imaginary or an outward sound, is the beginning of real superconscious chanting.
Gradually the subconscious repetition will change into superconscious realization, bringing the actual perception of God. One must chant deeper and deeper until the chanting changes into subconscious and then superconscious realization, bringing one into the Divine Presence.
Because by chanting a Mantra with sincerity and devotion, one is knowingly (or unknowingly) putting focus on the Mantra or on God. By putting focus on God, the focus on the World is lost. This leads one to detachment.
No longer concerned about the wins and defeats in life, one goes on merrily chanting the Mantra, putting the focus on God. Thus by forgetting the world, one remembers God.
From a jnanic perspective, this is self-realization (negating the unreal and realizing the real), from a bhakti perspective, this is madhurya bhava (unconcerned about the world and concerned only with God), from a karma perspective, this is vairagya (detachment to fruits). This leads to moksha.
Paramhansa Yogananda says in His works about concentration and its effect:
When a person by unshakable concentration can visualise any image or object with closed eyes, he gradually learns to do the same with open eyes. Then by further development of concentration, he can connect with God’s all-powerful consciousness and can materialise his thought into an object, perceived not only by himself but by others also.
Concentration consists in freeing the attention from distractions and in focusing it on any thought in which one may be interested.
There are also other benefits, like providing good karma by chanting, removal of bad karma, quick progress in spirituality, getting desires fulfilled, etc.
How am I to believe this?
(Just examples) Mrityumjaya mantra helps to overcome fear.
Gayatri mantra helps to become discriminate and detached.
Omkara helps to balance all states of the mind.
How is this possible? Is it like some kind of magic?
Most of the users here are computer scientists, i.e. they know how to code. So let's take a analogy of computers.
We write computer programs. When we write something like
printf ("Hello Dear Divine sparks");. The computer prints it out.
How is this possible?
The computer itself is incapable of understanding our words. It can only understand
machine code isn't it? Therefore in order to communicate with the computer we have
something called the programming language. When the syntax of the programming
language is properly followed, it is possible for communicating with the computer.
Using just coding, computers can be made to UNBELIEVABLE, SUPERB things, isn't it?
The same goes for the body. The body is like a machine, it can only understand machine-level language. Thus is it not possible to issue statements to the body such as 'become hotter' or 'become colder' and expect the body to respond to that.
In order to communicate with the body-mind complex, the programming language called the Mind is needed. The Mind is the programming language that helps to communicate with the body. Then who is the programmer? The programmer is the soul or jiva or individual consciousness or I-consciousness.
In order to instruct the body in an efficient manner, the syntax of the programming language must be used.
This syntax is the Mantra. By properly following the syntax of the programming language (don't miss any semi-colon!! or don't make logical error)
the computer's true inherent abilites can be used. Similarly by proper pronunciation, proper meaning and proper techniques,
the inherent abilities of the body-mind complex can be brought to unfold.
Sri Swami Sivananda on Practical Aid to Japa in 'Japa Yoga'
Pronounce the Mantra distinctly and without any mistakes. Repeat it neither too slowly nor too fast. Increase the speed only when the mind wanders.
Just as different programming languages have their own syntax, different languages have their own mantras. Thus sticking to the Sanskrit language for chanting Sanskrit mantras is beneficial.
How important is proper pronunciation and proper chanting for liberation:
However, this is only if you want the specific worldly and physical benefits of the mantra. For the one who desires LIBERATION, the good karma and other benefits or siddhis provided by these mantras are simply obstacles on the way to Self-Realization or Moksha.
In the Bhaagavata 11.15.33 it says:
The most wise ones say that those who are engaged in the cultivation of the sublime path of Bhakti (Devotion) or Giaan (Self-knowledge), and who are trying to link with Me (i.e., God), for them all these Sidhis are obstacle; because of these Sidhis, they end up uselessly wasting there time.
Thus the sadhaka who desires moksha does not chant in order to get these worldly benefits, He chants in order to get closer to God. Thus the first three benefits apply to the sadhaka desiring Moksha, while the worldly benefits, such as gaining siddhis (supernatural abilities), getting money, getting fame, etc. do NOT interest him.
Sri Ramakrishna says in The Gospel of Sri Ramakrishna:
People of small intellect seek occult powers—powers to cure disease, win a lawsuit, walk on water, and such things. But the genuine devotees of God don't want anything except His Lotus Feet.
For such sadhaka, it is not necessary to follow any strict rules for realization. For such a sadhaka, chanting the GOd's name with FAITH, FOCUS and LOVE is alone enough (although a Mantra initiated by the Guru will do Wonders on His growth).
Sri Ramakrishna says
Approach the Divine as a child approaches his mother, with the same purity, sincerity, ardent love, and faith, and the Mother will come to you! Call ‘Ma, Ma’; call again and again. The Mother is bound to come.
That is it is not necessary to use complex punctuations and tongue twisting incantations to call upon the Lord of the Universe Sri Krishna, all that is needed is Love and Faith.