As children, many of us are told to speak the truth as we see it but as we grow up, we realize that the forthright manner of asserting the truth can have undesirable consequences, as it may impede some higher ideals that need to be upheld or perhaps because there are conflicting ideals which may call for exercising discretion. In the spiritual path, one gradually becomes conscious that behind the objective affectation of truth stands a Higher Truth which needs to be understood and manifested in speech.
In letters to disciples seen below, Sri Aurobindo offered a few clarifications on the nuances involved in establishing “truth in speech”.
In the first place, there is a great difference between uttering as truth what one believes or knows to be false and uttering as truth what one conscientiously believes to be true, but is not in fact true. The first is obviously going against the spirit of truth, the second does homage to it. The first is deliberate falsehood, the second is only error at worst or ignorance.
If you want to be an instrument of the Truth, you must always speak the truth and not falsehood. But this does not mean that you must tell everything to everybody. To conceal the truth by silence or refusal to speak is permissible, because the truth may be misunderstood or misused by those who are not prepared for it or who are opposed to it − it may even be made a starting-point for distortion or sheer falsehood. But to speak falsehood is another matter. Even in jest it should be avoided, because it tends to lower the consciousness. As for the last point, it is again from the highest standpoint − the truth as one knows it in the mind is not enough, for the mind’s idea may be erroneous or insufficient − it is necessary to have the true knowledge in the true consciousness. 
According to folklore in India and perhaps in other parts of the world, it is said that one who always speaks the truth gains the power of prophecy (i.e. whatever one speaks will come true in the future). This is technically incorrect according to Sri Aurobindo; it is only the self-realized sage who is inwardly conscious of the occult Truth who is said to have the power of prophecy. Words uttered by enlightened sages are said to have the power of materialization because their identity is united with the Divine.
A sage also becomes conscious of the fact that something which is not true today could become true at a later time, because the future event has already been shaped in some occult world and can manifest some day in the material world. Therefore, a sage cannot deny something which is not visible today! As Sri Aurobindo elucidated in another letter:
From the point of view of higher Truth, it must not be forgotten that each plane of consciousness has its own standard. What is truth to the mind may be only partial truth to a higher consciousness, but it is through the partial truth that the mind has to go in order to reach the wider more perfect truth beyond. All that is necessary for it is to be open and plastic, to be ready to recognize the higher when it comes, not to cling to the lower because it is its own, not to allow the desires and passions of the vital to blind it to the Light or to twist and pervert things. When once the higher consciousness begins to act, the difficulty diminishes and there is a clear progress from truth to greater truth. 
Considering all these intricacies, one can (truthfully!) say that only one who is established in the Self can be said to be genuinely speaking the Truth. The rest of us are getting there but not quite there yet.
Related and unrelated posts
- Self-control over speech
- Distinguishing between stilling the mind and dynamizing meditation
- Developing discernment on which actions are spiritual
- The greater powers of the sense-mind (Manas)
- The latent Consciousness within Matter
- Syncretism in Sri Aurobindo’s thought – part 1
- Sri Aurobindo. Letters on Yoga, SABCL vol 24, Physical Transformation, p 1559.