Read more about Honesty from the works of the Mother and Sri Aurobindo.
What is Honesty?
It means Non-Deception
Truthfulness means first truth-speaking and to keep the speech in harmony with the deepest truth of which one is conscious.
One must know the Truth- be in touch with the truth of things, not merely speak the truth as his mind knows it.
Relation of Honesty with Other Qualities
Honesty, straightforwardness, loyalty and sincerity are closely related it is extremely difficult for someone to be perfectly sincere without being loyal and honest. 
There is no greater courage than to be always truthful. 
Truth is the characteristic of an upright man, the stamp of his nature. Love of truth makes him face all fears. One speaks as he should, no matter what happens to him. 
Honesty in Parts of the Being
Falsehood in the body is a falsehood in the CONSCIOUSNESS. The falsity of the consciousness has material consequences and that's what illness is! 
Vital honesty: not to allow our sensations and desires to falsify our judgement and determine our action. 
Mental honesty: one does not try to deceive others or to deceive oneself.
It is a mind that does not attempt to deceive itself. And in fact it is not an “attempt”, for it succeeds very well in doing it!.
Lack of Mental Honesty
When one begins to see that one has made a mistake, the first movement of the mind is to push it into the background and to put a ….cloak of a very fine little explanation, and as long as one is not obliged to show it, one hides it. That is “lack of mental honesty”. All these things are habitual, automatic, you are not even aware of them; When you discover this, you become aware that you are living constantly in a… “self-deception.” 
The Need for Complete Truthfulness
One must insist on a complete truthfulness in the whole being which will refuse to accept any denial of what the psychic discrimination sees or any affirmation or consent anywhere to what it disapproves. 
Truth and Divine Truthfulness
Truth does not insist on a blind adherence to the spoken word—it demands that a man shall cleave to the principle of Truth in things only and in certain cases the principle of Truth would demand that he should break his vow and not keep it. If a man pledges himself to something that is against the principle of Truth... it is not Truth to keep that pledge—for it would be a pledge to follow falsehood and truth cannot be rooted in falsehood, 
Honesty Indispensable Basis of Yoga
The indispensable basis of our Yoga is sincerity, honesty, unselfishness, disinterested consecration to the work to be done, nobility of character and straightforwardness.
Common Misunderstandings about Truthfulness
Something one believes to be true—which probably was true for a time—on which you partly base your action, but which, in actuality, was only one opinion. 
Honesty and Fear
When a person is forced to speak something, that does not make him truthful as at the very next moment fear may drive him to speak without frankness. 
Why is Truthfulness Necessary?
Victory over Falsehood
Each acknowledgment of error is the demolition of one of the lords of Darkness. It may be an acknowledgment to oneself if it is absolutely honest and is no subtle regret apt to be forgotten the next moment and without the strength to make an unbreakable resolution not to repeat the mistake.  Absolute truthfulness must govern life if one wants to be close to the Divine.
Role of Honesty for Transformation in Different Parts of the Being
There must be self-restraint, entire truthfulness, a constant thought of the Divine in all one does. This is the way for the change of the lower vital. By constant self-dedication and self-discipline the Force will be brought down into the external being and the change made.
Truthfulness in the Whole Being
If the will to be true to the inner self in all ways is strong and persistent and vigilant and always calls in the Mother's force, complete truthfulness can be done. 
Role of Honesty in Sadhana
To serve God under a cover is easy it and insists on complete service, fearless service and honest service. 
The opposition in certain parts of the being exists in every sadhak and can be very obstinate. Sincerity comes by having first the constant central aspiration or will, next, the honesty to see and avow the refusal in parts of the being, finally, the intention of seeing it through even there, however difficult it may be. 
If one is honest, sincere, spontaneous, and especially when experiences come without any effort on your part to have them, and as a spontaneous expression of your deeper aspiration, then these experiences carry with them the seal of an absolute authenticity. One must not deceive yourself. One must be sincere and honest with a complete inner rectitude. 
One’s service to the Divine must be scrupulously honest, disinterested and unselfish, otherwise it has no value. 
How to Develop Honesty?
The greatest liar can be the most truthful person. One must not despair if one finds in himself/ herself the greatest weakness, for perhaps it is the sign of the greatest divine strength. You are "like that" because, precisely, you ought to be the opposite. And difficulties are there just so that you may learn to transform them into the truth they are hiding. 
One must cultivate control over speech, Control over one's speech is more important than complete silence. The best thing is to learn to say only what is useful in the most accurate and truthful way possible.
Process of Cultivating Mental Honesty
This means that an effort is needed in order to be mentally sincere. There must be an effort, there must be a discipline. It is acquired by a very constant and sustained effort. It sometimes takes years. Time must pass, one must have changed much within oneself, ones vision of things must have become different, one must be in a different condition, in a different relation with circumstances, in order to see clearly completely how far one was deceiving oneself- and that moment one was convinced that one was sincere. 
One must make an effort to have a quiet mind, see that this mind surrenders and becomes perfectly honest and later to be concentrated on.
How to Inculcate Truthfulness and Honesty?
Children should not wait to be grown up before they learn to be truthful: that cannot be done too early; and to remain truthful, it is never too soon to acquire the habit.
One must set the children the right example. Be yourself what you would like them to be. Give them the example of equality of temper which makes children free from fear, for what makes children deceitful and untruthful, and even cunning, is the fear of being punished. If they feel secure, they will hide nothing and you will then be able to help them to be loyal and honest. 
Sincerity, honesty, straightforwardness, etc. are things that are taught better by example than by beautiful speeches. You will see that little by little your child will reflect this ideal in himself and spontaneously manifest the qualities you would like to see expressed in his nature. 
One can increase in stature only by being truthful, sincere, obedient and grateful.
Content Curated by Arpita Joshi
Read more about Honesty from the works of the Mother and Sri Aurobindo.